I think it's good that you let loose in a game instead of in "real life."
I don't play MMO's as I find them too addictive (and boring, to be honest), but when I played a little in the past, I preferred PVE (player versus environment) instead of attacking other players. As you noted, some people take their game very seriously and even though I don't think that's very healthy, I don't want to attack a player and cause him to be truly upset.
I think you have a lot of power in your mind and if you wanted to, you can stop "liking the fact" that your actions piss people off. The question is -- do you want to?