I think kids bully online because there isn't that immediate peer feedback to learn from that you get in real life. When you push someone around in real life, you might get some uncomfortable looks from your peers, which gives you some idea that what you're doing is not right. On the internet, there is nothing like that, so you never learn not to bully.
Mokalus of Borg
PS - Then again, you might never get that feedback if other kids don't know it's wrong yet either.
PPS - It's just a theory, and I'm not a psychologist.