The mind and body are less separate than most people seem to think. Changes to body chemistry can greatly change how, and even what, we think:

• Alcohol is well-known for its disinhibiting effects and also for changing one's perception of what makes an attractive potential partner ("beer goggles").




• I have type 1 diabetes and occasionally become significantly hypoglycemic. At times, this can make it nearly impossible to think about anything other than "Eat! Eat! Eat everything in sight!" as my body tries to raise blood sugar to an acceptable level.




• Trans friends have described being fully aware of their taste in potential sexual/romantic partners changing significantly when they started on hormone therapy.




• Although I haven't looked into it in any detail, several studies have suggested that changes to gut bacteria can produce significant changes on a person's state of mind.




• "Here we report that, although rats have evolved anti-predator avoidance of areas with signs of cat presence, T. gondii's manipulation appears to alter the rat's perception of cat predation risk, in some cases turning their innate aversion into an imprudent attraction. The selectivity of such behavioural changes suggests that this ubiquitous parasite subtly alters the brain of its intermediate host to enhance predation rate whilst leaving other behavioural categories and general health intact." (Fatal attraction in rats infected with Toxoplasma gondii., emphasis mine)




• Cordyceps fungi compel infected ants to climb to a specific height, clamp on to a leaf stem, and stay there until they die. (Admittedly, this example is a behavior, not necessarily a "thought process" as we normally understand the concept.)

Given these phenomena, would it really be that surprising for the biological changes associated with vampiric transformation to alter the subject's perception of "food", "humans", and how those two categories relate to each other?

Actually I don't think that there is a lot of change of mind involved.
Just look at your points:

• "Humans are cattle, vampires are the master race." That is something we already do. (Or most of us do) We think that our species is best and that we have the right to use inferior species to our liking. We breed and eat all kind of animals without having a bad conscience. So when you change you species, humans are now a different species than yours, so the same rules apply for humans as for animals.




• "It's natural for vampires to attack humans, so there is nothing bad about it." It's natural to eat other species, there is nothing bad about it.




• "Humans will die someday anyway, so why bother about their lives?" [Edit: Don't get attached to cattle, you will see it die] Pets have such a limited lifespan. Usually they die way before you. So don't get too attached to your pets.

Pretty much the same mindset.

Rationalization

In psychology and logic, rationalization or rationalisation (also known as making excuses[1]) is a defense mechanism in which controversial behaviors or feelings are justified and explained in a seemingly rational or logical manner to avoid the true explanation, and are made consciously tolerable—or even admirable and superior—by plausible means.[2] It is also an informal fallacy of reasoning.[3]

Any vampire that doesn't find a way to rationalize their behavior will die, either by suicide or starvation. No need to come up with strange biological or supernatural explanations - their attitudes are completely normal for human beings in that situation.

Changes in Brain Chemistry

The transformation causes a change in brain-chemistry on a deep level. Empathic feelings which are usually evoked by other humans, are now only evoked by other vampires. This leads to vampires not seeing humans as the same species anymore. They will see other humans like another animal race and will be affected by human suffering no more than by the suffering of a pig.

The changes seem to be rooted in the different pheromones, which are produced by vampires. This smell causes vampires to regard other vampires as kin, while humans smell like livestock to them. Other factors are a subtle change in mimics and micro-expressions. A vampire will smile and cry differently than a human. This subtle cues will make a human smile look slightly wrong and "inhuman" to a vampire. Within a short time, the vampire will disassociate with the inhuman figures and cannot help to feel the wrongness in their mimics, gestures, voices and smell.

This will lead to traumatic experiences in many vampires suddenly unable to feel connected to their former human peers. These events are not unlike a dissociative episode, which causes severe psychological trauma in the victim. This can lead to highly egoistic and even psychotic behavior. But over time most of the vampires will develop coping mechanisms and seek new bonds within vampire society.

Immortality is not living forever, immortality is everyone else dying. - The Doctor

It's going to be really hard to maintain your human values when individual humans seem to be such a brief flicker in the passing of time. What does it mean to shorten a human life by a little, they barely exist long enough to register their lives at all.

And that's for someone who might have started out fundamentally a good person, people who become vampires aren't necessarily so good in the first place, so it's not so far to fall.

Long Term Changes

These are the easy ones to explain. Given time, our personalities gradually change. I'm a very different person than I was even 20 years ago, even if many of the key aspects of my personality from back then are still present. Were I to live another 50 years, or 100 years, my personality would doubtless transform further. If my life changed drastically, as becoming a vampire would do, these changes would presumably be accelerated.

Not only that, but the parts of my personality that remained constant would have been formed in a society that no longer exists in the same form, and so my personality would diverge not just from the human I was, but from the personalities of those humans who were born and raised in these later societies.

Short Term Changes

Other answers bring up possible changes in biochemistry, as well as psychological factors such as rationalizations of necessary vampiric behavior, survivorship bias, and the possibility of power corrupting. I would add a variant of that last one: they were already corrupt, but have only recently acquired the power to act on that corruption.

"Humans are cattle, vampires are the master race."

"It's natural for vampires to attack humans, so there is nothing bad about it."

"Humans will die someday anyway, so why bother about their lives?"

Radical egoism

With fairly minor modifications (mainly replacing "vampires" and "humans" with "my group of humans" and "some other group of humans"), none of those ideas are as rare as we might like. What is rare is for people to have the power to act on those ideas in a significant way. Many war crimes are simply normal crimes, committed by people who are prevented from doing so during peacetime because the parts of society that would stop them are more powerful than they are. When society no longer holds them in check, their personalities may appear to change dramatically, but what has really changed is their power relative to those around them.

Vampirism doesn't cause people to become evil. It reveals those who already were.

They change, but because society makes them.

The transition from human to vampire is gradual and horrible. As you crave for blood, people start to notice something is weird about you, and start to push you away. Eventually, they realize what is going own, and you're forced to run away from everything you once called home. As time passes, your human self becomes more and more a memory of the past, destroyed but humans themselves.

Some vampires isolate themselves and simply drink blood to survive, while others want vengeance and to bring upon humans what they themselves have suffered. This way, you can explore the spectrum of reactions and experiences the vampires have gone through.

Vampires don't die from old age

When a person changes into a vampire, their personality doesn't actually change at all. The only real difference is that they now have a hunger that can only be satisfied by drinking blood.

When people think about vampires, we are subject to suvivorship bias - any vampire who refuses to drink blood won't live long, so we only see those that give in to the hunger for blood. Additionally the more bloodthirsty a vampire is, the more people they will go after. Of the vampires that survive, the more ruthless ones are more likely to be encountered.

So why do people think that new vampires undergo a massive change in personality? They simply don't understand how someone could go from a normal, average person to the bloodthirsty creature that might hunt people for sport. This leads them to reason that there must be something about the process of becoming a vampire that significantly alters the personality.

However, recall what it was like to be a small child. It's rare for children to understand their parents - they say things like "I'll never be old and boring like them". Adults do a better job, but it's still common for them to not understand people who are older than them or those who have lived a significantly different life.

What we forget is that vampires don't die from old age. With vampires, we have both of these obstacles to understanding them - not only have they lived far longer than us, they've also lived a significantly different life during that time. The transition from normal person to bloodthirsty monster is gradual, but because vampires do not die from old age most vampires are already old enough to be on the monster side of that transition.

There is my take on this problem.

1. Natural selection. Vampires who couldn't (or didn't want to) change their attitude toward humans commited suicide or starved to death, while survivors seriously rethought their values and attitudeds. Or found excuses that they themselves try very hard to believe into.




2. During conversion their astral body visits astral plane where they gain spiritual and philosophical insights about life. These insights make them adopt ruthless (toward humans) life philosophy.




3. There was changes in their brain on physical level that turned off their ability to experience affective empathy. They can understand emotions of humans, but they don't feel them.

Power Corrupts

Not explicitly stated in the question but by implication of being able to prey on humans and considering them an inferior species it's not unreasonable to think that vampires have a distinct advantage of some kind. New vampires might try to maintain some semblance of their former humanity but gradually come to internalize superiority, perhaps initially they think in terms of leadership or patronage and eventually devolve into disdain as they interact with inferior beings.

See the Stanford Prison Experiment for an example. Edit: worth pointing out the experiment has questionable scientific value. There is some indication power correlates with negative traits like dishonesty but of course real life is messy. I'd argue the evidence is sufficient for fiction.

A practical example

Think about Lord Elrond in The Lord of the Rings. Pretend like you don't know the story, and I tell you that there is a character who:

• Holds grudges over singular mistakes that were made generations ago and actively dismisses the future generations of their ally's plea for help because of it.


• Overgeneralizes other races. If one member of the other race makes a mistake, all others are obviously going to do the same.


• Prohibits his child from dating someone from another race and living in the conditions of this other race.

You wouldn't particularly peg this guy as the good guy, right?

What makes Elrond not evil is the fact that he's effectively immortal, which means that Elrond has a different view on time-sensitive issues.

• Elrond was there when the mistake was made generations ago. To Elrond, that's something that happened in his life. He forgets that humans have had several generations since then and that the descendants of Isildur cannot be blamed for Isildur's mistake.


• Elrond did not immediately spring to action when called upon, because he has no sense of urgency. As an immortal being, he wouldn't be particularly bothered about taking 50 years to do something, whereas humans will effectively live their entire lives in that timespan.


• Elrond cannot process the idea of fleeting mortality when it affects him. 50 years is the blink of an eye to him. And his daughter has chosen to give up her immortality and will live the rest of her life in the blink of an eye. To a being with a nigh-infinite lifespan, this is the equivalent of Arwen telling Elrond she's going to kill herself in the "near" future. Effectively, Elrond is a parent who will see their child die and thus lashes out emotionally at that prospect (blaming Aragorn for what is Arwen's choice).

The point is not to deeply analyze the character of Elrond, but I wanted to show you how a character who is numb to old age behaves differently (or with different interest) than a character who does age.

The general approach

How can I explain the radical change in personality when a person has been transformed into a vampire?

In general, a person responds to things that positively/negatively impact them. Often, people do something they don't innately want to do because the benefits outweigh the drawbacks.

• A religious person may choose to follow their religion's commandments (even the ones they disagree with) because they want to avoid eternal damnation.


• An old man who very quickly breaks his bones will learn to avoid physical activities. Even if he really likes physical activities, he wants to avoid the pain.


• A young person can refuse to smoke or partake in unhealthy behavior because they want to elongate their lifespan.

But when you take away the punishment, there's no need to still do the thing:

• When the religious person finds out there is no eternal damnation (heaven for everyone, or no afterlife for anyone), they may stop complying with commandments they don't agree with.


• When the old man get an unbreakable skeleton, he may pick up physical activities again.


• When the behavior does not shorten their lifespan, the young person no longer has a reason to avoid the behavior.

This can be gradual or immediate, based on the character. A good guy will hold on to their morality, but they may lose sight of it over an extended period of time. A bad guy will almost revel in the opportunity to do the thing they can now freely do.

However, you should choose the right response for the right character. Each of your given examples carries a subcontext of the person making this argument:

1. Superiority is a fact we cannot deny and should not avoid.


2. This is how nature works.


3. The long-term goal outweight the short-term goal.


4. No one is more important than me.

They are all valid justifications for vampiric behavior, but you can't just give any character any justification (unless you're specifically going for a "you never know what a person is truly like" vibe with your story).

Just to oversimplify, you could stereotypically assign these justifications to:

1. A white supremacist


2. A biologist or survival expert


3. A cold and logical person (we come back to Elrond here)


4. A vain politician or celebrity

This is despite them being humans once and still having memory of human life.

I always think back to Sophie. She was a classmate of mine. In class, there were a few advanced students who did not need test revisions (scoring over 90%), but a lot of others did need guidance (scoring under 60%). Because of this, the teacher made a rule that during test revision, those scoring >85% could instead play a board game.

Usually, this was the same 5 students. But Sophie was an infrequent addition, she always scored around the 80%-90% mark.

• When Sophie scored <85%, she complained that it's unfair for some students to not have to do the same work as her/the rest. She was the one to loudly complain.


• When Sophie scored >85% and someone else made that same argument, Sophie responded that she clearly didn't need revision and shouldn't be made to do it. Again, Sophie was the only one to take center stage to defend her opinion, the other "passing" students were relatively indifferent to it compared to her.

People's opinion on categories changes when people change categories. This is normal human behavior, even if an outside observer can plainly see the hypocrisy.

Someone who is actively protesting tax cuts for the rich (because they are poor) might, when becoming rich themselves, start arguing that the rich shouldn't need to pay more taxes than other people.

Or, a human who advocates enthnically cleansing the world of vampires, may let go of that notion when they themselves become a vampire and possibly start arguing the opposite.

This is effectively what Magneto does to Senator Kelly (an anti-mutant political leader) in X-Men: he makes him a mutant, and Kelly changes his rhetoric overnight because he now fears becoming subjected to the system he tried to put in place.

Rejection and Acceptance

Although I whole heartily agree with Dave Sherohman that physical changes can have extreme impacts on the mind, I want to add an additional source of personality change.

When someone becomes a vampire, they will likely be rejected by their community, and then accepted into the vampire community. A change of social circles, along with a change in identity, can vastly change someone's behavior.

Some examples of this include cults (see Jonestown tragedy), the military (teaches you that you should kill these people), the Nazis (ditto), religions (not necessarily a violent change in this case), or even just migration (migrants often assimilate into the culture they move into, if accepted).

If the vampires teach you that killing someone is okay, and you feel a biological urge to eat them, I see no reason you would not eat them.

In fact, although a change in brain chemistry definitely can change personality, it is not required. If you make your vampiric narcissism purely a social phenomenon, you could very plausibly have "nice vampires" that were not rejected by their communities, and only feed on cattle. Anywhere in between also works too: for example, maybe some vampires respect humans, but require human blood, resulting in a gentleman's thief type personality. Or maybe their family donates blood to them (in the form of bags so the vampirism does not spread). Or maybe they do not have a physical need for human blood, but a strong psychological urge, that some vampires try to resist. Or maybe human blood is not required at all and is just a vampire society thing, but becoming a jerk automatically happens when you become a vampire either way (for biological reasons that Dave mentioned).

Since you did not disallow supernatural explanations:

In Bram Stokers Dracula, it was implied Vampires where literally damned souls, compelled to walk an unchristian path (Dracula himself was very much at peace with his final death, since his soul was finally set free). That's why vampirism was a curse to. It was kind of like being in Hell, except you were stuck in your corpse, and it didn't matter how you had lived your life.

In the same story, vampires aren't very functional as people. Dracula himself was an exception. Every other vampire in the story were mentally much more limited than him, almost as if they had sustained brain damage (or become slaves to a malignant force). Dangerous, but more like how an animal, like a snake or a rabid dog is dangerous.

Dracula himself... well in life he was supposed to have been a massive genius with an absolutely brilliant mind. Even then it took him 500 years before he managed to conceive of his grand scheme. That was how long it took him to more or less train his mind back up to something close to what it had been in life. And that was unique as far as vampires went. Every other instance were more or less "Drink blood. Hide from the sun. Repeat."

Two options come to my mind.:

Only psychopaths become vampires

The psychopaths already have some sort of brain difference that makes them not able to feel empathy. As such they are pretty much predestined to easily adapt to the new role. In fact, they often already believe they are superior to other humans. You can already refer to them as not having feelings (or heart if you prefer more poetic description).

The blood thirst enforces will of survival

This is actually something you may learn from real examples. Consider this tragedy in Andes. Despite moral concerns those who survived turned to eating human flesh. Of course it was about eating the deceased ones, but still in extreme circumstances it might turn you from your normal moral objections.

You do not have other food. What do you do then?

In the Bible you can read:

When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I set aside childish ways.

Paraphrase it by changing child with human, and man with vampire.

When I was a human, I talked like a human, I thought like a human, I reasoned like a human. When I became a vampire, I set aside human ways.

If you change, also your ways and standards change to match the new status. It's perfectly logical. It's part of the process of change. You don't become a man/woman just because your body changes with puberty, you become such because also your mind changes.

Same goes with vampire.

Incidentally, the same line of thought is followed by Major Kusanagi in Ghost in the Shell, just applying it to cybernetic bodies, not to vampires.

When you grow up/evolve, you change.

humans eat other humans in desperate situations (los andes's airplane accident for example, there is a movie "survive!" from 1976). if you become a vampire you are not "less moral"or you have a change in your personality, you are still the same but you didnt knew that you would be able to do it, until it happens. you are just another creature that will do what you have to do to survive.

Change in the philosophical foundations

Our worldview and all of our personal philosophies are based on various foundations, including the fact that we are mortal, and our other biological needs. For example, your perception of mortality might be that you need to leave a mark in the world so that some part of you never dies. But if you're not mortal, your philosophy towards leaving a mark might immediately disappear. You might have reproductive needs that lead you to conclude "I must work on my personality, skills, social value, etc. to improve my chances of reproduction." If your reproductive needs suddenly change, then you might no longer care about these pursuits of self-edification. The vast majority of your actions are governed by fundamental needs, laziness, or need for amusement.

If a person's fundamental needs and foundational facts of their existence are suddenly changed, then it has a cascading effect in changing all of their personal philosophies and actions. Immortality would predictably trigger a significant personality change.

Parallels to hunting

I would also conjecture that anyone who hunts / cooks / and eats animals when they aren't forced to would make a good psychological model for a vampire. What life experiences led them to this hobby? Did they tag along with parents for their first hunt? Do head vampires ever take newly converted vampires along for a first hunt, preaching all sorts of philosophy about how killing humans is okay, talking about the sport, and so on (probably only in situations when a head vampire likes the person they converted)? Some fiction writes that vampire victims are only converted when the vampire chooses them to be; whereas the others just die. If you take this approach in your fiction, then this means that there are almost no vampire converts that wouldn't have the direct mentorship of a veteran vampire, and guidance on a first hunt.

Hunger pangs

If vampires don't drink blood, then do they die? If so, then some of them will quickly die, especially if they don't have an instinct to know why they're wasting and dying. Alternatively, it might be the case that no matter how long you go without drinking blood, you never die, the pain just keeps getting worse and worse. If this were the case, then converted vampires would always be forced with the choice between drinking blood, enduring infinite agony, or committing suicide. By the time they reach this extreme crossroads, they might be suffering enough and desperate enough that they would be ready to make extreme moral compromises. Psychologically, this would parallel people who give-in to torture and reveal vital intel, the Tell-Tale Heart short story, people who take unethical jobs out of financial desperation, instances in which depression leads to suicide, instances in which people have turned to cannibalism, and the time that Aron Ralston cut-off his own arm because he was intractably trapped between two boulders.