Here's the deal - anything can spark off someone who is disturbed and thinking about violent action. Most of the time the people involved in these shootings are quite intelligent, but lack the ability to apply their knowledge to an appropriate outlet. Guess what one of the scientifically proven benefits of playing games is... that's right, problem solving and application of learning. Several studies done about the effects on games have already been done, and they all come up with a similar conclusion; that games help cognitive thinking, problem solving, etc. In fact, a recent study has shown that whether or not you game, and how much you game, are leading indicators of ability as a surgeon. It is because gaming, in particular FPS, teach precision and visualization skills that are difficult to acquire by any other means. Skills don't discriminate. So, a kid who is bullied or otherwise feeling ostracized, sits listening to music, watching TV and movies, feeding their view of themselves and the world, and playing games that while also feeding that beast makes them better critical thinkers, problem solvers, etc. So the depressed kid goes from being fairly intelligent but unable to apply that textbook knowledge to broader activities, indoctrinates themselves with a violent mission, and has now trained their thinking towards execution.
Violent people will seek ways to act violently. Anything can inspire a violent person to act violently. Murder has existed as long as humans have. Blaming music, movies which are new relative to span of human existence, and games which are even newer, is at best misunderstanding the basis of the problem. I happen to think that guns are likewise not the primary culprit. But we better think about limiting guns way before we start limiting games, movies, and music. A crazed person may still find a way to kill, but it's a lot harder to go on a killing spree with a knife than it is with a gun dumping out dozens of rounds in a few seconds.