The problem here isn't that the game is M-rated, it's that game/film/music classifications have no effect when they aren't enforced. When a parent allows their children access to restricted content then they are taking a shit on the classification system designed to protect their children from unsuitable content. This point is too difficult and unpopular to argue however so it's far easier to simply target the artists (I include game designers under that banner) and push the blame onto them.
There's another issue here. This problem wouldn't exist in Australia. Why? Our citizens aren't armed to the teeth. It is exceedingly difficult to purchase a firearm here and automatic and semi-automatic weapons are illegal for all, regardless of what use you might have for them. The process of acquiring a licence is lengthy and involved so, short of those in the Olympic shooting team, most don't own firearms at all. The illegal trade of firearms is not sufficient to create too much of a problem so, by and large, we don't need to worry too much about our police officers being shot down by "kids warped by video games." Perhaps if Americans (and I know I'm generalising here) didn't carry wallets in their left pockets and guns in their right then the problem wouldn't be so large. Again, that's a difficult and unpopular argument to make so let's just blame Eidos.