There's one big problem with the argument that he could have shot them in the back and had nothing to worry about. There's nothing in the story that indicates the other two were even armed. Had he shot the first one, and then the other two turned and ran (which I believe they most likely would have), and he then shot them in the back, and they turned out to be unarmed, I think there's an excellent chance he would've ended up being charged with manslaughter.