Television is contributing to violence. When children and other impressionable people see that people on television have license to do violent things to others, they begin to think that violence is a good way to resolve their problems in life. They then turn to violence as a way to solve their problems.
Televised material has a vast range of programs that can significantly influence its audience. Violent programming is no exception. Although television programs are subjected to rating systems, it's physically impossible for the ratings board to ensure that ONLY the intended audience watches the program. Moreover, violent programming is more likely to attain more views; conflict sells.
Watching violent tv doesn't make people want to go out and rape, pillage and murder everyone in sight. You don't mug someone because you saw it on tv, you do it because you want what they have. That doesn't make it ok, but still. I doubt tv has that much of an influence on whether you are violent or not.
I know it may be hard to believe, but violence existed long before television was invented. We were doing a pretty spiffy job of inventing new ways to fold, spindle and mutilate ourselves for centuries before the video screen came to dominate our lives. The fact is that people who blame TV for such things simply do not understand the world and wish for a scapegoat.