While I agree that computer games do not make you any more aggressive or violent than any other media, I do think playing a lot of shooters does one thing to the mind: it learns to abstract the human shape as a target. This doesn't make you want to kill or something, but I think, if you ever found yourself in the situation where you had to, it would probably be easier for you to pull the trigger if your brain had already learned this abstraction. I don't really have any concrete information to back this up, though, just some observations I made during my military service.