People who claim that violent video games have 0 effect are wrong, however people who claim that it is the cause of all the world's problems are also wrong. The problem isn't just with the video games that people play but with the instruction as to right and wrong that they receive. So as it is I agree with Allanon here, letting your kid play violent games isn't necessarily a bad thing but you need to help him know that it's just a game and actions taken in the game aren't all right in the real world most of the time. (I say most of the time here because some games do have positive concepts in them mixed in with all the killing)
I'm thankful that my folks kept me a little farther away from violent games when I was younger, it helped me turn out to be the man that I am today.
Ring
P.S. I'm a little sad that Mavrick beat me to the grammar police job but he's right.