I think we are more attuned to violence because of news media and politics, not movies and video games. Tragic events are sensationalized by both news and politics and analyzed ad infinitum.
People with mental issues and lack of self control cause violence. We all lack self control now and then. We make poor decisions when we lack control. Emotions get away from us and before we know it we've said or done something we wished we didn't. Without self control it doesn't matter what you watch or play when you lose your patience, sanity or temper.