I wouldn't be concerned, he is seeing things on the news and reacting to that. Boys are all about adventure, and with all the toys available that "shoot" something for whatever reason, he just sees it as play. He cannot grasp the concept of death as anymore than someone falling on the ground. Don't make a big deal of it, he will change what he wants to be when he grows up more times than you can count. If he were to continue to talk about killing people, then it would be of concern, but I don't think there's a problem.