Personally i think it would be ludicrous to believe shooting games don't have an effect on people.
A fact that isn't up for debate is that watching violence/ negative things we aren't accustomed to seeing desensitizes us in those subjects.
Take the scariest horror movie you've ever watched, find the part in it that just makes your hear lurch when you see it, finish watching that part, rewind, watch it again about 20 times, and see if it gives you the same emotional effect as it did the first time you watched it. Try watching it a week later, i'm fairly sure the scene won't be as scary as it was in the first place.
Does this mean your going to become a serial killer when you grow up? Not necessarily, but it sure isn't going to the most empathetic being on the planet. On the other hand, if you already are unstable and fighting with yourself for a reason why you shouldn't become a serial killer, playing a couple games of GTA might just give you that push off the edge you need.