Most of them
Dio was abused as a kid, but he rejected his humanity so fuck him.
Kars can't even be judged by human standards.
Kira was just born fucked in the head, but his murderous impulses were probably encouraged by his father.
Diavolo's ass was a playground when he was only a kid, so you can't really blame him for how he ended up.
Pucci was an absolutely pure boy who had to deal with the trauma of watching his sister suck his brother's cock and then kill herself.
Valentine... had no dad I guess, there's not really much reason why he's nuts.
Jobin is a manchild who was bullied as a kid and emotionally manipulated by his mother.