Better discuss this with your lawyer or someone else familiar with the laws of your state. If the crook is trying to run away and you shoot him, you may be prosecuted for murder. It is a dicey situation since, as you pointed out, he could turn around and take some shots at you. If he is still in possession of a weapon when you shoot him, that may mitigate in your favor. But if he is unarmed when shot, you might be in deep doo-doo.