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Good and evil are man-made concepts that don't exist in nature, so therefore the answer is simple:

Nature magic doesn't make you evil, it makes you amoral(evil and amoral aren't the same thing) because nature itself is amoral

Nature magic simply makes the caster more like itself. You're right that it'd strip the user of his/her morality, it'd probably remove what you'd call humanity as well. A channeler thus corrupted essentially goes feral, with zero conception of morality and as much restraint on their base impulses. If they're hungry and they see Bambi they'd kill and eat it with no hesitation. If they're horny and they see a member of the opposite sex...you know what happens.

On the other hand, that also makes them incapable of malicious intent. For example, prolonged torture(whether for a specific purpose or just for cruelty's sake) is a man-made behaviour that channelers gone feral would never do.

To cast a spell, one must use their soul as a channel for the flows of magical radiation. Because it is normally a weak background field, the spell involves gathering weak streams of Tham and binding them into larger ones, before releasing the stream according to the specific pattern that will produce the desired effect.

The stronger the spell, the larger the stream needs to be, hence the more Tham is going through one's soul at any given point in time.

The streams can be gathered separately by several people, dividing the quantities needed, but this requires passing Tham from one person to another in order to produce the same pattern. This leads to some unavoidable leakage at each transmission, which is why a spell is more powerful when cast by a single person.

When fluxes become too important, they start to exceed the capacity of one's soul. Although the specific amount varies from one to another, the eventual effect is the same: when one's soul is overloaded, Tham accumulates and stretches the soul out, until eventually the connection to the person's mind is damaged.

The longer the exposure to strong streams, the more damaged the soul will be, and with it, the less empathy the person will be able to experience. Deprived from empathy, they would only seek their personal benefit and become unable of altruistic behaviors.

At some point, when the soul is damaged beyond repair, the person does not even feel any difference in casting extremely powerful spells, for their soul has been stretched and twisted in too many ways already.

TL;DR souls are soft pipes for Tham. Less pipes is less leakage, but more stress due to larger fluxes. Without soul, you have no incentive to behave morally.

Doing this would make the spell more powerful because they do not need to spread the energy that they channel with others in the circle. The downside to this is that it will eventually twists and corrupts you, making you evil and stripping you of morality.

How could nature magic be a corrupting force?

Nature magic doesn't corrupt you, because Nature is neutral (just like the tiger than eats a child isn't malicious, it's just hungry).

You corrupt yourself through the pride -- and soon arrogance -- of being able to do something that others can't do themselves, and the temporal power that allows you to accrue to yourself alone (by not needing to share the spoils with anyone else).

As many answers have already stated, nature itself is neither good nor evil. It is amoral.

So my suggestion would be that the energy doesn't per se corrupt the caster it just turns them towards their natural state.
Now if you were cynical you could say that all humans are, at least latent, evil and corrupt. Everybody has a dark desire that lingers somewhere in their subconscious. Thus the energy they channel turns them more and more towards their natural, evil state of being.

As for why that only happens when someone channels the energy all alone there are two options. You could go with either a certain threshold that needs to be passed in order for the change to actually occur or you have it affect everybody interacting with the field and it's just very weak when the effect is spread out between multiple people and grows exponentially for a single caster and is therefore much more noticeable.

That also opens up the option for really purehearted individuals that can cast by themselves without being corrupted. Of course only for as long as they don't have any selfish desires at all.

I saw something alike to this at one of R.A Salvatore books, which was I guess on the Cleric Quintet series.

In the book there was an evil god and one of her servants went beneath the temple and activated the curse of the god.

Affection of the curse is any whom exposed to it goes mad in strange ways. One of the affected was a druid whom had high skilled in the nature art. He turned himself into a bear (which is a high druid spell) and he neither could able to revert into human form nor he was aware of himself. His great powers worked against him.

With this, he attacked his companions, did some violent actions against people who don't deserve it and so. And slowly sucked into madness and violentness.

So after a brief brainstorm you can go with something like this: The beauty of the spell itself convinces people into relentless casters whom takes heed of the magic art as liches or warlocks do...

More so:

When casters cast the spell together they will not affected by the maddening effects. Because they see through each others mind and could see how all the other casters hold on to. But when a person alone perform it, he/she couldn't able to picture/echo which idea is right and which is wrong.

More so person would lose their moral code which they held onto with all of their lives. More they cast this spell alone, more their actions become relentless and more cruel to any who in relationship with this person. And at one point they become someone who do anything to learn more with the nature art. So they expose to unavoidable maddening caused by nature magic art.

ALSO:
On DnD (forgotten realms) magic works through the Web of Magic of Mystra. Any spell you cast is exist on the web of magic and struck the target within. Both nature spells, holy spells and arcane and more so, uses the web of magic. This web exist on some other plane... Maybe you can go somewhere from this.

Brainstorm is the feed of art of our world.

This is perhaps a lot simpler then you may think...

Any activity done by anyhuman could be considered a selfish act. they are doing these things for their own benefit.

Some examples of this are easy, we work to pay for food, a home, and enjoyment, we don't work for free, no one grows crops for free, it is all to further themselves. whether alone or in a group it doesn't really matter.

Now i can already hear the comments, what about charity... well that's an unfortunate result of humanity, we have something others don't, that makes us feel the need to help. not selfish obviously... well, what happens when we help, we feel good, we feel happy to have helped someone less fortunate. we do it to stop ourless feeling guilty.

I can still hear the comments of people saying that they don't do it for those reasons, but i challenge anyone to name a situation where a deed was done that did not benefit the doer in any way shape or form...

That Challenge Aside

So Magical or not, natural or not, in a group or solo... everyone wants to stop what they feel is wrong and encourage what they feel is right, to do that they need power... and what does power do... it corrupts.

Also the simple fact that while one party might see their actions as good, another will see it as evil. Just look at Thanos in the MCU, he truly believes himself to be doing the right thing. so its easy for magical power to corrupt but the channeler themselves may not even see it as bad.

TLDR Power Corrupts

The downside to this is that it will eventually twists and corrupts
you, making you evil and stripping you of morality.

How could nature magic be a corrupting force?

Good and evil are human perceptions of the interactions of natural forces life and death (nature).

In its natural state nature energy exists with its opposing magic death energy. By performing the magic the user channels immense sums of natural life and death energy but only expends the nature energy on the ritual.

The user is then left with an immense amount of anti-life energy. Being now heavily influenced by this anti-energy the user would seek to correct the imbalance from the perspective of the anit-energy. To them its not murder, its the need to return balance to the universe.

You could even go so far as to describe them as transitioning from a being of life to a being of death or undead. Much like how the living view the undead with disdain so to do the undead view the living.

Magical forces necessarily come along with hazards. In some cases it might be attracting the attention of a great demon or simply blowing yourself up, but nature magic is more insidious.

The great danger with natural magic is magical disease.

In much the same way that a fire or an icicle are quite sterile, elemental magics are also sterile. As a swamp is full of life: so natural magic contains a plethora of magical bugs, parasites and diseases, any one of which could twist it’s unfortunate host in a number of unpleasant ways. Dipping into the swamp over and over again without getting properly cleaned up and checked out between dips just makes it that much more likely you’ll catch something nasty.

Now: elemental magics will just kill you outright if you don’t respect their power. Natural magic won’t, but the magical diseases that will infest you if you don’t have sufficient protection (say a second circle of Druids acting as an occult antiseptic) will seek to breed, replicate or multiply. Like a cold will make you sneeze: magical diseases will drive you to greater and greater acts of natural magic. They’ll line you up and aim you at situations where you’ll cause the greatest harm simply in order to try spread themselves.

Worst of all: just like a cold you won’t be able to stop the sneezing. You’ll be entirely aware that your sanity and morality are slipping away but you’ll be as unable to keep hold of your soul as a rabbit with myxomatosis is able to keep hold of it’s eyesight.

Basically: natural magic is icky. If you’re going to take a dip: use protection or you’ll catch something awful.

The energy is channeled through the frontal lobes. It gradually destroys them in the manner of a frontal lobotomy.

Nature magic energy is not bad in and of itself. It is just one kind of energy. But the frontal lobes of a human are not meant to take that kind of energy, and channeling so much energy through them destroys them little by little.

These wizards do not become evil in the way of Satan - scheming and purposeful. But they do become less human, amoral, disinhibited. Corrupted, as the site where their humanity is stored wears away. It does not take place the exact same way for each person. One real world example of this is a frontal lobotomy.

https://sciencebasedmedicine.org/frontal-lobotomy-zombies-created-by-one-of-medicines-greatest-mistakes/

A psychiatrist named Myerson presented lobotomy to the Sternburg
family as a way for them to keep Bennie at home. He explained that it
would disconnect the signal from Bennie’s emotional centers that told
him someone was dangerous, so he would no longer be violent. After
much discussion, they bowed to the authority of the expert and
authorized the surgery.

Here is what was done to Bennie: holes were drilled in his skull; the
blade of an instrument was inserted through the holes, its handle
swung as far and deep as possible.

He was no longer violent, and the family no longer had to fear him;
but he didn’t speak a word, he barely moved, and he didn’t react to
anything or anyone. He was incapable of taking care of himself and
required constant supervision. He had eruptions of inappropriate
sexual behavior with family members. He would do odd things in public
like whirling on the sidewalk like a dervish in a slow trance. He even
had to be reminded not to swallow food whole without chewing. After 15
years he suddenly recovered the ability to speak but then subjected
the family to a surrealistic nonstop flood of fragmented thoughts. He
had become “a head without the czar inside.”

You can read about the behaviors of other persons with frontal lobe damage to give you ideas for your story - frontal lobotomy is the most extreme but frontotemporal demential is another, and brain tumors are yet another.