That's what characters like him are telling me. These guys can't be trusted, they're antisocial, misfit nutjobs and can't exist without freaking out or harming people, they're scary. I don't see anything complex, they lack empathy and scruples due to some kind of chemical imbalance.
Kotomine was born a cold, evil mofo, and he died as one. Hell, you could even make the argument that he never really 'lived', and his death was just a really, really late abortion.
A Hollow, violent man who died hollow and violently and made a lot of people suffer because no one had the good sense to off him while the offing was good.
Those who are mentally ill because of biological rather than psychological issues are a lost cause, and should be locked away or disposed of as soon as possible. It's not like they're capable of minding, only the normal people dumb enough to care about them would.