I grew up in a happy household, my parents were both highly educated, and my siblings and I are all relatively successful. I don't think my parents even knew about my bullying because I was really good at making sure I didn't get in trouble for it.
I didn't bully everyone, just particular kids. The best way to describe it is that I saw weakness in some kids, it sickened me, and I wanted to crush it. I didn't do it to make myself feel better, or because I had self esteem issues or to make myself popular. I was amongst the top students in my class, had a bunch of friends, and even kind of nerdy so it wasn't like I was a stereotypical jock picking on nerds.
I can't explain why I felt this way, I just did. And to be honest, I can still feel these feelings sometimes around certain people, but I just stifle them because I know it's wrong. So I think it must be genetic and something I was born with because despite me knowing it's wrong and I don't want to act on my feelings, when I interact with some people I still get the same feelings of "God this person is weak, I want to crush them."
This is why I recommend people fight back against their bullies. If the person I bullied punched me in the face, not only would I not want to get into a fight (as I mentioned I never physically attacked anyone and never got into fights because I was really good at picking kids who wouldn't fight back) but then I would reconsider whether they were actally weak or not.