"I don't like you. Die!" isn't a snide remark, it's being childish. It's not roleplaying a psychopath, because even a fictional villain has to be interesting to the audience. That is why Irenicus in BG2 doesn't isn't written like that; he is murderous and without any qualms or remorse, not a teenager throwing a tantrum. Imagine what would have happened to the BG series if Irenicus would, instead of giving his famous speeches, just proclaim "I don't like you. Die!" every encounter with him.
