Violence in media ABSOLUTELY causes people to be more violent. No question. Don't believe me? Here's my two word proof: Cake. Boss.
I challenge you to watch three episodes of the TLC show about a New Jersey baker and not desperately want to stuff pieces of impeccably decorated sponge into your cake hole. It's impossible. I've watched the whole series and some days I'd kill for a piece of cake. Media is just that powerful.
It's not just me and it's not just cake. The poker world experienced a huge boom in the early 2000s because Matt Damon made it look so cool in Rounders (if I get good at No Limit Texas Hold'em I could sleep with Gretchen Mol!). Glee led to record numbers of show-choir auditions (if I get great at singing and dancing, I can sleep with Cory Monteith!). And MSNBC undoubtedly makes people want to go into politics (if I become a senator I can sleep with people who aren't my wife!).
But this isn't new information. You've known it for a while. Dangerous shows constantly tell people not to "try this at home." Why? Because if it looks fun on TV, people are more likely to do it.
I don't think we need to make violence in media illegal; I'm a philosopher, not a politician. But it isn't doing anybody any favors when we pretend it's harmless. It's clearly not. TV makes it seem cool to shoot guns at people / aliens that look like people / zombies that used to be people we love. Healthy people can resist that pressure but not everybody can. For me, it's easy with guns but not with cake, which is why I remain unarmed and overweight.