Personally, I hate seeing kids playing such violent games, especially when they 'shoot' at others not involved in the game - to me that is threatening and just mean.
There was some research done on this that led to the book & documentary, Raising Cain. The upshot of it was that, psychologically, it is important for boys to be able to play such games in a safe environment - i.e. under supervision. When they are not allowed this, they have to find other ways to learn about violence & their own roles in society etc and this can lead to teenage fights, gangs & violent adults.
This is not my opinion, just what the researchers found but it was interesting to see.
I know some kids are just not interested in guns etc. I just hope my little'un is like that!