Rowling treats her mentally ill characters like crap. Draco Malfoy is one of the only characters to be very explicitly shown to have depression and she treated him like complete trash. Characters who suffer from paranoia are just a punchline to her. Characters who go “crazy” are clearly just evil, because only crazy people do bad things!!!!
Unless, of course, it’s her favourite characters, who somehow manage to go through some ridiculous things (12 years in Azkaban, having to kill the person you love, having all your friends die, literally dying yourself, living with a piece of someone’s soul inside of you, literally being mind controlled by someone) and they all somehow end up as perfectly balanced adults because they’re too strong and pure and kind and perfect to let anything affect them. Silly me. Of course only evil people can be mentally ill.