I read an article once that discussed the infamous Call of Duty "No Russian" stage, where players have the option of participating in an airport massacre or not, and apparently some behavioral health professionals were saying that the kids who didn't partake in it had some clear issues with separating fantasy from fiction (which could lead to behavioral issues down the road) while the kids that just shot everyone up had a normal grasp on reality and understood it was just a game.
Now I don't feel so bad about running everyone over in GTA V...