Small children don't know what death is. It is always hard to explain, that their father will never come back. Elder think up lot of stories like "father is far away", "he is a spy in enemy country", "mother is sleeping" and so on. Death is too complicated concept for fragile mind.
But there is no real life without death. Death always surrounds us. Doing all this murders, they just explore death. And exploring death, we open new pages of book of life. Practically all children cut themselves with a knife, because they don't know what is "sharp" really is. And burn their fingers in fire, as they need to know - what is "hot".
So i think all this happens not by reason of TV, or violence in computer games. Contrary, these children knew nothing about death. So this was just exploring of the world, real life. Just curiosity.