Seeing violence in the media can contribute to a child's violent behavior. If nothing else, watching fights and bloodshed in movies disensetizes children to violence. There is a reason cruel dictators often had their children attend executions. After seeing a man beaten up or killed enough times, the child is no longer shocked by it. It becomes no big deal.
When children are surrounded by violent video games, they will copy that behavior. If their friends are interested in violent actions, they will be even more inclined to become involved. Parents are often very unaware of the video games, movies, and TV shows kids watch. They are learning straight from their peers.
Kids are learning at young ages so they go by example. What they see is what they do. They aren't mature enough to know what is right. If you told a kid to jump he might say no. But if you say its fun and jump with them thhey are more likley to join you
If violence is shown to kids with both explanation or morality as to why it is wrong, then of course it may cause some problems. But luckily media such as the news show violence as an awful thing that shouldn't be done. I have seen violence in the media for a good 3 years (I'm 13), and the way it is portrayed in most forms of media makes it clear the horror of it. Just hearing about such things as Islamic State in the news makes me feel that the world is wrong to be like it is.