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Not all demons go berserk and insane.
The insanity happens to those who resist the corruption of infernal energy. The greater their resistance, the more insistent infernal energy becomes in corrupting them. It's for this reason why the angriest demons used to be the rational and strong-willed people before they were corrupted.
Those who didn't resist the corruption, especially those who embraced infernal energy and their demonization, were spared from the insanity.
They are allowed to remain 'intelligent' and civilized but forever changed through and through. Actually, the 'sane' part of them was debatable given how a newly transformed demon's instinct was to kill and destroy.
The only real difference between them was the fact that those who embraced their demonization remained aware of what they were doing and could control their actions.
But of course, they aren't limited to this.
