Their personality is a direct product of their environment. Shit parents make shit kids, it's nothing short of a miracle when a kid somehow sidesteps that fate and comes out a better person than their parents.
Kids can be raised in exactly the same way and turn out vastly different. My wife is pretty similar to her siblings. I only share a couple of personality traits with my sister, and as such we’re very different people.
Most of the research points to nature being way more heavily weighted than nurture. Most people don’t want to admit this. For parents it’s hard because you feel powerless. For everyone else they want to attribute their good/bad traits to how they were raised instead of attributing it to biology.
Because if you agree there'd variance, then you must agree that some kids would have more bad behaviour then others.
Also, do you think mental illnesses are binary?