if someone is under 15 and playing a game thats m15+ its up to the parents to say what they should and shouldn't be playing. Ratings are given to movies and games because they may have unappropriate subjects.
I saw that documentary on the columbine shootings and they partially blamed Marylin Manson because the kids listen to his music. U can hardly blame him for what they did.

It depends on the kind of person, although violent video games can sometime go one step too far. To be honest its not the matter of playing shooting games for the violence, its just the gameplay that makes it fun. When the old pacman games and things like that were popular, people played it just for fun(and the basic fact that they were the only thing around at the time).

I think if people play it in a reponsible manner no gets offended.