Many of God's people were fighters.
David was 'A man after God's heart'. He wasn't allowed to build the temple though, because he was a warrior. His son Solomon completed that task.
Every little thing we do counts. When you choose to do good instead of evil, when you choose to help a person.. when you choose to say something nice instead of just being grumpy.. we define our character as we go, and our character is the only thing we can take with us from this life as we move on.
Who am I to say you should or should not play violent computer games?
Learn what's good for you, and most importantly, include God in whatever you do.