When it comes to online interaction, I think it's safe to say the John Gabriel Greater Internet Fuckwad Theory holds some truth behind it.
You give people a barrier of anonymity and they get comfortable, sometimes too comfortable and then you can see some of the real nasty sides of people out there.
I've noticed that often the GM's in MMOs like to take a non-interventionist stance on a lot of the negative behavior. More often than not they'll tell you to just put someone on /ignore rather than dealing with the individual.
Demographics can't help too. In the case of FPS games, I'd imagine many of the players are males who can be described as being aggressive. If not in the real world then certainly in-game. Those kinds of people can be majorly annoying to those in the game who aren't like that.
Jam it back in, in the dark.