While hated by most, these people are typically non-social for reasons beyond their control, and manage to get that one glimpse at society by eliciting responses from forum-goers who, nine times out of ten, think they know everything and that anything that goes against the norm is bad. There are of course the assholes who do it just for fun, but a lot of these people are suffering: people with manic (or often just plain old) depression, people with real issues in real life, or people who's real life isn't as cupcakes and unicorns as people on the internet assume.
I say this because I know people in real life who many would consider trolls, and they "troll" on forums because they receive no attention from their family and very little from anyone else. They are saints for putting up with it in real life, but it'd be unhealthy to pent up all their frustration. So what do they do? They speak out on the internet, often in obnoxious and annoying matters. The very fact that people respond to what they say is important to them, just because they get satisfaction in knowing that some people somewhere acknowledge their existence, and perhaps more importantly their opinion. Believe me, most trolls you see aren't "a poor excuse of a human being". Their lives in real life is worse than many could imagine. Everyone needs to vent sometimes.
Troll: WOW U FUCKS NARUTO IS FOR HOMOS ONLY.
Forum-goers: U poor excuse for a human being you need to die
Troll (in real life): Man, I don't really like Naruto. But no one knows that. No one in my life give a flying fuck about anything I say or do.