This isn't a topic where I can tell you what to do, you have to choose for yourself. I can just try to give a few insights and then I'm going to refer you to a video that helps with some perspective from the Video Game Industry's side of the matter. You are welcome to Google the other sides opinion, there's plenty of it out there.
This is an age-old question; Does violent play make a violent child? My grandfather played cowboys, my dad played cops or Superman. My kids grew up being Ninja Turtles, G.I. Joe and Luke Skywalker. No matter how hard a parent tries to keep their kids from playing what they see as a violent game or toy, kids find a way to play good guys and bad guys. Clearly this doesn't cause our children to grow up as anti-social miscreants or society would have become a Mad Max world long ago. I believe it just helps our kids understand right and wrong and to have the opportunity to be a hero and save the day.
Recently there was a terrible event at an elementary school where a young man, armed with guns from home, went into a school and murdered several children and teachers. We always ask how such a thing could possibly happen. I would hope that the answer stays where it should...the person who committed the crime was not well! Personally I think the weapons were too easily accessible to him, but that is my own opinion. Even then, the guns didn't make the young man shoot people...he made the choice. I think when something this awful happens we want a definitive answer...it had to be his music, a movie or video games. It was none of those things. It was a guy who was clearly unbalanced.
There are all levels of violence in video games and there are video games that are too violent for me. Therefor, we don't have those games in our home, because that is my choice. I don't like horror films or explicit Rap music either, but I DO like playing video games where I am shooting, I like suspenseful and scary movies and I like a lot of Rap and Hip Hop. I have NO deep desire to hurt anyone...I don't even believe in spanking! I have friends who do like those more violent forms of media and not one of them is in any way a mean or violent person. I have family members who have made the choice to not allow shooting games into their home. Its a personal choice for each of you.
Obviously there is no easy answer to the violent acts some people commit, I just hope that people will do their research before condemning any one type of media, take what they have learned from that research and then do what they feel is best for themselves and their family.
Here is the promised video...Violence In Video Games ...Its from Gamespot's "What If Machine" series and it has some excellent points and insights to consider. I will warn you that there are images of some violent Video Games in this video (the kind we never buy :p) but the research and experts give perspective and excellent insights for you to consider about your kids and gaming. (You might want to move up to about 1:50 in the video to avoid the worst)
GameSpot is a great site for reviews if you want to know if a certain game meets the standards you want set in your home.