Paul Graham weighs in on trolls, and what an online forum can do to minimize their corrosive effects. Here's one of the causes of trolling:
The third cause of trolling is incompetence. If you disagree with something, it's easier to say "you suck" than to figure out and explain exactly what you disagree with. You're also safe that way from refutation. In this respect trolling is a lot like graffiti. Graffiti happens at the intersection of ambition and incompetence: people want to make their mark on the world, but have no other way to do it than literally making a mark on the world.
Here's Gresham's Law of trolls:
... [T]rolls are willing to use a forum with a lot of thoughtful people in it, but thoughtful people aren't willing to use a forum with a lot of trolls in it. Which means that once trolling takes hold, it tends to become the dominant culture.
A previous post on trolls (with some links) is here.