Griefers
In the Game Girl Advance blog, I found a link to an article about Griefers - that is, players who thrive on making life intolerable for other players of online games.

I think all who play or chat online have met "griefers" in one shape or an other. It is a version of flaming, annoying behaviour in order to stir the tempers and make people lose their composure. It gives the griefers a sensation of power and control when their victim is annoyed or hurt. The sudden interest in griefers comes with the advance of commercial online games. Since the sport of griefers is to make life hell for others, it becomes very unpleasant to stay where they are. This costs the game-companies money, as people don't want to play when they are harassed. The griefers are also often clever hackers, who put their energy and time not into playing the game by the rules, but into circumventing the rules. This ties the resources of the game up in a constant battle as the programmers try to build mechanisms and close loopholes for undesired (read: unprofitable) behaviour.

While I as a player detest griefers, the scholar in me understands why it's possible to say that they make the game better. They cause tension and introduce an element of the unknown into the controlled game universe. By hacking or just using the game creatively, they expose flaws and weaknesses in the design, and they stimulate the other players into gaining skills and strength as a defensive activity.

The problem is that they are so horribly annoying, cruel and inconsiderate, and they act that way regardless of who their victim is. Griefers are the bullys of the games, and the bully-mentality isn't any prettier online than offline.

PS: Mark Bernstein supplies this with his comments on griefers, or as he calls them:snerts, and their influence on the software industry in general.