I have to concur with video games making people violent, especially as the graphics get better.
Some people already treat video games with more importance than real life, but adults really aren't the problem, it's kids. When you let your young, impressionable 7 year old child play GTA or CoD, it desensitizes them towards violence, and instead of being repulsed, they don't really care. Add a bit of natural crazyness, and boom, you have a mass murderer, if he gets the chance.