I've both been the victim of griefing, and in deathmatch games been a griefer myself from time to time.

I've never griefed an MMO before, i think people that do that are just sociopaths. FPS deathmatches are different for some reason - i dont know. It's the all-out chaos of the thing, there's death everywhere, it's complete anarchy and nihilism anyway, and it just feels pretty good sometimes to sit perched on top of something and rain grenades or snipe into a hard-pressed battle below….