I've thought a lot over the last couple weeks about the problem of trolls. It's an old one, as old as forums, but we're still just learning what the causes are and how to address them.
There are two senses of the word "troll." In the original sense it meant someone, usually an outsider, who deliberately stirred up fights in a forum by saying controversial things.For example, someone who didn't use a certain programming language might go to a forum for users of that language and make disparaging remarks about it, then sit back and watch as people rise to the bait. This sort of trolling was in the nature of a practical joke, like letting a bat loose in a room full of people.
The definition then spread to people who behaved like morons in forums, whether intentionally or not. Now when people talk about trolls they usually mean this broader sense of the word. Though in a sense this is historically inaccurate, it is in other ways more accurate, because when someone is being a moron it's usually uncertain even in their own mind how much is deliberate. That is arguably one of the defining qualities of an idiot.
I think trolling in the broader sense has four causes. The most important is distance. People will say things in anonymous forums that they'd never dare say to someone's face, just as they'll do things in cars that they'd never do as pedestrians—like tailgate people, or honk at them, or cut them off.
Trolling tends to be particularly bad in forums related to computers, and I think that's due to the kind of people you find there. Most of them are more comfortable dealing with abstract ideas than with people. Hackers can be abrupt even in person. Put them on an anonymous forum, and the problem gets worse.
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.
The final contributing factor is the culture of the forum. Trolls are like children (many are children) in that they're capable of a wide range of behavior depending on what they think will be tolerated. In a place where rudeness isn't tolerated, most can be polite. But vice versa as well.
There's a sort of Gresham's Law of trolls: trolls 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.
Ok that is my take on the situation.