Like anything, limit their time. Make sure they play outside 3 or 4 times more than video games.. even more if you can get them out there.
Talk about reality verses pretend. be serious that you want them to understand the consequences of real life military, real life violence.
Talk to them about laws about how we behave as society and about how each and every person has loves, has things they are good at and ways that they help our society. everyone loves their mothers as much as they love you, so to lose their mother to violence.. is devastating.
If they are old and mature enough in your eyes to play these games, they will be old enough and mature enough to consider the real life consequences of this behavior in real life.
Remember, this is not a one time conversation.. this is a conversation that should continue through out their adolescents also.
These games are only going to become more intense the more skilled they are, the more they will want to play.. But keep them living in reality..
Our daughter learned about death while in preschool, when one of the mothers was killed in an accident. In elementary school one of the dads was killed in the military. we spoke about what this meant to their families. The sacrifice this dad had made and the sacrifice this family would always live with.