You may not like my answer, but try to curb, or all out stop, the viewing of violence. Whatever it is that a person watches or does, if they do it constantly then they become desensitized to it. It starts to lose meaning.
If all I did was watch love stories and listen to love songs for years on end, I would stop being a romantic and start treating love in a blase' manner because it would lose it's luster from over-viewing. The same with violence. I don't think the kids who do this really see it as something that is wrong. They don't really see it as anything at all anymore because they grew up with it on T.V. and video games.