We're fucking MURDERING PEOPLE for chrissake. They deserve a reason. Revenge is a good reason. Equipment is a good reason. Because of the game we're playing, I don't like your voice is a good reason. But when you walk down the road and are trying to kill somebody, for all your character knows, this could be the last time he ever sees them. If you're a murder gnome, get some PTSD from fighting on the front lines. If you're a goodie, have a nervous breakdown about killing (acolytes, I'm looking at you), if you're evil, have a reason for killing someone.
The person you just murdered has a reason for playing this game, and though you don't have to tell them that you're going to kill them, bash first and then tell them that you killed them just to prove a point. Then don't listen to what they say.
It's fucking weak to kill someone and ignore target just because you don't feel like roleplaying. You certainly felt like ruining someone's con, they deserve a fucking word or two.