Another thing to consider is that correlation does not equal causation. Violent people probably like violent media, so of course someone who goes on a killing spree will probably have seen a lot of violent movies and perhaps owns some violent video games. This does not mean the violent movies and games caused them to be violent. This concept has been drilled into my head by my psychology professors over the last several years (correlation not meaning causation, that is).
I think that if they're going to increase the warning label content for games then it must also be done for movies. Game ratings have a surprising amount of information as to the content of the game compared to movie ratings. Plus, the game rating info is usually way easier to find than the movie info. A game like GTA IV or whatever will say rated M: violence, profanity, nudity, alcohol/drug use and so on quite clearly on the box. A movie like Kung Fu Hustle will say rated R, and then in tiny print may have something along the lines of "violence, language, etc", but it's way harder to find that stuff on movies. Finally, I find that even when I can find those bits of info, the ones on movies tend to be way broader than those on games. Meaning that nudity in movies could be a brief flash of breasts, or a nearly hard core rape scene. Games tend to be a little clearer on the content.