Andrew your asking on a gaming website if it's ok to play violent games, as most people here clearly do it's unlikely that many are going to say that it is wrong.
The people you should be asking are your parents.
Anyway here is a little bit of info & advice.
The meaning of being a christian is to be an imitator of Christ. You have to ask yourself
would christ do this?
Would he be happy for you to pretend to shoot someone for the fun of it?
How do you feel when you play it? Agressive? Angry? Happy? how does it effect you?
The ask yourself about the content of the game, does it have any demonic aspects etc.
It's a grey area and so I understand it isn't easy, but whatever you decided you must make sure that in your heart your ok with it, and that you really do belive God is ok with it too.
In the end it's between you and God, thats a private relationship and what you decide could affect it.
Like in every hard choice do this.
Pray to God.
Read what the bible has to say on the subject and be honest with yourself
Discuss it with your parents and older people you consider "wise" in your faith.
If you really think a game is simply bad and your friends want to play it with you. Say no, if they are friends they will learn to respect your beliefs.
Saying no to the minor things now makes it easier to say no to the major ones later on in life.
Stick up for what you belive just like you did on this board ;)
"..just keep on trying 'till you run out of cake"