I’ve never really thought games could make people violent. However, I will say that it might not be merely a question of violence; rather, it depends on how the violence is handled.
I wonder if anyone ever did a study comparing, say, people who play Metal Gear or Fallout to people who play Duke Nukem or Manhunt. All of those games are rated “M”, and contain violent and disturbing scenes, but the former handle them in a mature, intelligent fashion. The latter, not so much.