This is a follow-up to "Often, enemies really are innately evil." I waited a few days before posting this because although this is true, I really wanted the message to sink in that many people, given a choice between torturing someone for 50 years and throwing dust specks into the eyes of 3^^^3 strangers, they would choose both. Denying this has done direct harm in a lot of people's lives including mine.
However, I should give the rest of the story. The main bully I had was very cruel, but after several years they suddenly stopped. Not overnight, but it was weirdly quick. Not only to me, but to the other people they also bullied at their school. And the change stuck; he never went back to it. Today he's quite a nice person.
There were a few other changes in his life around that time. He got really into World of Warcraft. He got really into church. But these don't seem likely to be the cause; I don't think a post titled "Why superstimulus and Christianity are the key for moral improvement" will be posted anytime soon. He attributes his change mainly to building good, constructive friendships with the people there. This is more reasonable; it is possible for good friends to inspire each other to do better. I'm not sure this is sufficient though. It was a very major change.
I can't say he just "grew out of it" because a lot of evil people remain evil as adults. My evil cousin "M", at least for now, is still evil. (Know how hard it is for a woman to be prosecuted for abuse, and lose full custody of the kids to the father? She managed.) I once caught an adult trolling in real life. I asked why they did that and he said causing suffering is fun. (I still consider this innately evil) One of my teachers tended to be casually mean to his students: for example, on a Monday he said that something is due "next Friday". I asked if he meant the Friday in 4 days or the one in 11 days, since sometimes "next Friday" means "next week's Friday". He mocked me in front of the class for not knowing what "next" means, and said it was the Friday in 4 days and that I was stupid not to know that. (Then later he said "next Wednesday" in a context that meant the one in 9 days). I will always think of him as professor Snape. Becoming good clearly isn't reliable.
So why did it happen? I don't know.