If taken to an extreme, it can even turn into Online Erotic Humiliation where the abuse turns into sexual arousal. So the next time you tell someone to go fuck themselves, you might have just given them the material to do so.
But maybe more common than that is good ol' Attention Seeking Behavior, which every single person who's ever spent just one evening with a child will be familiar with. The outcast, like the child, knows that hate is not the opposite of love, apathy is. All that negative attention is still attention, and the abuse is still several steps better than being ignored.
So Why Does it Happen on the Internet?
So we've established that when you say, in person, "Jimmy, go away, you're a retard" that Jimmy is just happy that somebody used his name and acknowledged his existence. Even if the only reason you used his name was to tell him to go die in a fire"Who's a super fag? Jimmy's a super fag, that's right. Go Jimmy!"
But when you type it on a message board, it's that much better. This isn't just attention, but attention that's being broadcast around the globe via the World Wide Web. The "we hate Jimmy" thread on a popular forum might be read by thousands of people. If that many people are reading about him, he must matter (hell, think of all the TV personalities who have made a career out of being hated).