There are 2 types of Troll:
The "Fuckwad": The moron that will spam insults without any true purpose for the sake of simply getting attention, he realizes the internet is an audience and he's motivated by the same reason stupid people are motivated to be in reality TV shows, except he knows that the internet will protect his real identity in the case of doing something truly stupid.
The "Subtle" Troll: This is the guy that you probably won't hear about after making his initial début, but that returns to feed on the anger he has created. This is the guy that drops in on a forum, creates a well-written and well-constructed negative review of a popular game (glorifying negative aspects but not by going overboard and revealing that he is a troll) this will then lead to the exposure of the WORST people in the fandom of said game, people that go: "OMFG YOU SUX SO MUCH! U CAN'T SEE THIS GAME IS AWESUM!".
The end results
The "Fuckwad" Troll is a nuisance, something temporary and mildly annoying, but not a threat to any community.
The "Subtle" Troll will make the community ashamed of itself, for it will bring out the very worst within it and expose it, and he'll never get blamed for it.
PS: Personally I'm rather fond of "Subtle" Trolls as they help the community improve itself and notice it's own flaws. Of course, sometimes, they destroy "social lives" as well by exposing the worst side of specifically vulnerable targets, but if a "girl is exposing herself on webcam while under-aged" or "a boy is posting his home address on Facebook", the subtle Troll will merely expose these flaws to the person (sometimes causing emotional trauma)... one might say for the "greater good"