Some people would argue it's about childhood trauma, and how the people trusted with taking care of children are often violent, teaching the future-adult that to get what you want/need, you have to be violent.
I don't think it's the only cause, but it may be one.
In my case, I was bullied at junior high and teachers didn't do anything, so there was a point where I snapped and took on the entire class so brutally and unexpectedly, people started fearing coming near me. No friends before, no friends after, lol.
I was also bullied in pre-school, where I learnt that if I hit him first, the teacher only notices him hitting back, so I got to hit the bullies and make them look bad. I didn't like it much, so it took the entire class in junior high to make me snap again.
I had to be sent to a school for problematic children (mostly retards) after I noticed I could harass my teachers and the system wouldn't do anything in that case too. After that it was chill because retards didn't pick on me.
On high school I was bullied again, but by then I wasn't strong enough to take on the whole class so I just dropped out.
The reason for bullying usually was me getting high grades, and showing off how proud I was without the social understanding that people hate when you show off. Now I try to be nice, and being nice online was what got me a great girlfriend when I was 17.
Sorry for the long text… just wanted to let it off my chest.