There are plenty of psychopaths and sociopaths among humans, and the internet gives them a means by which they can act out their worst inclinations anonymously.
There is a category of online game players, for example, known as “grief players”, whose sole goal is to ruin the game for everyone else. South Park did an amusing episode on this with regards to World of Warcraft, where a mystery player had gotten himself to a very powerful lever, and just went around killing all of the lower players for the hell of it.
You can find griefers in EVE Online as well.