I could have told you this without the benefit of any costly study.
In my case, in addition to the cruel kids, I also had a couple of gym teachers who bullied me, too (and a softball coach who deserves his own special level in Hell.) I try - often unsuccessfully - to imagine what kind of sad home life they must have had to have brought out such evil. But, usually, it doesn't work because I think they didn't really have sad home situations that caused them to treat the fat kids poorly. Frankly, I think a lot of people are just jerks and weasels when it comes to mocking and torturing other humans. Hey, after all, it doesn't have any lasting effect on the abusers to make other people feel like crapito, and they get a good laugh out of it.
To bullies, I say this: remember, while Hilter may be getting a pineapple shoved up his ass at noon each day in Hell (by Satan himself), Hitler's probably assigned to do the same to you, but with a durian fruit.