I'm 21 now. I have been playing video games, including M rated, for most of my life. I've watched my mom play Tomb Raider, Duke Nukem, and Doom over her shoulder, and I've played my fair share of M rated games myself from Mass Effect to Halo to Call of Duty. I love the stories they tell- They're interactive! However, it takes a sick and twisted person who is obviously already mentally unbalanced to go commit crimes- Crimes that they probably would have already committed eventually whether they saw or played violent media anyway.
That's why I really don't understand why it's so hard to stop and look at the individual's mental state and why they were raised without that supervision from their parents.