mjhjmtu

Could it be

Depression?

Clues:

Most lines seem more poetic than directly descriptive, but they all seem to be describing the same ideas:


"I tear you apart and leave you like a shell." - People with depression report being unable to feel happy or sad, like an emotionless shell.


"Desperate, you'll try to kill me, only for you to succumb to me." - Suicide is often a consequence of depression. Obviously, since the cause of depression is one's own brain, trying to kill the source results in one's own destruction at its metaphorical hands.


Also the acrostic INSIDE refers to the illness being entirely mental or internal.

Are you

You conscious, or your inner mind?

I am one of your greatest enemies, I tear you apart and leave you like a shell.

You are your own worst enemy.

Now I end you, one of my specialties; torture and debase you, similar to hell.

You only know what makes yourself tic.

So tremble in fear against my might, I am the stuff of nightmares.

Your mind give you the nightmares.

I am an endless decay, a blight, here to expose your affairs.

Your mind knows what is troubling you.

Desperate, you'll try to kill me, only for you to succumb to me.

???

Evil incarnate, you're forced to see, I'll ignore all your pleas.

You can never-not think.

Acrostic

INSIDE, you mind.

Bit of a stretch perhaps but are you

Sleep?

I tear you apart and leave you like a shell.

Sleep separates you from your senses. While you're asleep you're similar to a dead body or just the shell of a man.

Now I end you, one of my specialties; torture and debase you, similar to hell.

Sleep can also lead to nightmares which can be considered tortuous by some.

So tremble in fear against my might, I am the stuff of nightmares.

Again, nightmares is heavily associated with being asleep.

I am an endless decay, a blight, here to expose your affairs.

Bit stumped on this one. Only thing I can think of is that sometimes people confess their secrets while talking in their sleep.

Desperate, you'll try to kill me, only for you to succumb to me.

Everyone needs to sleep and no matter how long you try to fight off the need to sleep, you will eventually succumb.

Evil incarnate, you're forced to see, I'll ignore all your pleas.

Again not too sure on this one. Perhaps just a continuation of the above idea; no matter how much you fight, sleep will come for you.

It might be

Anger

You were scared of it as a child(parents being angry).

You now see it in others as an adult

And its consequences leave you devastated

I'm assuming

Anxiety, because it unravels you (Tear you apart and leave you like a shell), causes suffering and potentially a breakdown (now I end you, torture and debase you), it directly causes you nightmares, exposes your affairs, and attempting to defeat it can exasperate the anxious mind.

Could it be:

Bacteria?

I am one of your greatest enemies, I tear you apart and leave you like a shell.

Harmful bacterial diseases make you sick, and kill living tissue.

Now I end you, one of my specialities; torture and debase you, similar to hell.

People can die of nasty diseases ("end you"), and even if they don't, it can feel like hell.

So tremble in fear against my might, I am the stuff of nightmares.

As per above

I am an endless decay, a blight, here to expose your affairs.

Biological decay is caused by bacteria. Ultimately, flesh rots and the inside of a body is exposed.

Desperate, you'll try to kill me, only for you to succumb to me.

We take antibiotics to help the body fight/kill the bacteria.

Evil incarnate, you're forced to see, I'll ignore all your pleas.

But some infections ignore treatment.

Hint:
In your childhood, you feared me, as an adult, you recognized me in others.

Children fear getting sick. Adults recognise others when they are sick.

If you see me, you shall flea; I'll tear and break down this world with my brothers.

(I presume "flea" should be "flee" - otherwise this is an entirely different ballgame.) People do their best to avoid harmful bacteria when they're aware of its presence. Ultimately, all living things get consumed ("tear down this world") by bacteria (who exist in their countless trillions ["brothers"]).

I see the answer is Demon but my first thought was depression, then I thought drug or alcohol addiction.