The Washington Post's Max Fisher presents evidence that should cast doubt on the idea that violent video games cause violence:
The U.S. is an outlier in gun-related murders, even though we're not exceptional in how many video games we play. If virtual violence caused real violence you'd expect to see some kind of a relationship between the two, but in this graph there isn't much of one at all.
At the Incidental Economist, Aaron Carroll digs up more evidence that there isn't a link between gaming and violence, drawing on a few different sources.
It's possible that depictions of violence or brutality in video games lead to aggressive behavior. Even more likely, it's easy to believe that video games, not necessarily violent ones, drive social isolation and other undesirable behaviors among serious gamers. Gaming critics, however, need to account for the fact that the evidence points against a link between playing video games and violence.