I realize that this being Slashdot and this being a bully thread we're going to get a lot of "woe is me" replies, but on the flip side kids don't become bullies just because they wake up and decide it'd be a fun way to pass the time. Based on my childhood sample, they do it because:
(a) It's the conflict resolution method they learned at home, either from dad beating the hell out of mom or both parents laying into the kid, or
(b) They're screwed up emotionally, often from abuse, and as children don't have any idea how to deal with it.
One of the bullies who caused me (and a lot of other kids) a lot of grief in middle school turned out to have been molested by his uncle. Another lived in a trailer park and had no mom and a dad who was constantly falling down drunk and, I suspect, physically abusive to him. These are just the two I know for sure about, I'm betting that this isn't some wildly exotic circumstance.
In retrospect, I'm the one who got off easy. What those poor kids had to deal with was far, far worse than having their books slapped out of their hands or the occasional bumps and bruises.