- The criteria for deadly force do not depend on whether the person's back is to you or not. In general, if you have a *reasonable* fear for your life or another's, you can fire. You don't have to try to apprehend, give a warning yell or warning shot. You just fire. In this case, a person who has already committed a violent felony against you and is still close enough to harm you again would meet the criteria. It isn't what he does after pulling his pants on or what he plans to do, it's whether your fear is reasonable.
- Why did you shoot him in the back? Because that's the way he was when I was finally able to reach my gun. Next question, counselor.
It would be a much different situation if he had left the house and you were able to lock the door behind him, but went out and shot him as he was leaving. Depending on the state, you may be in trouble.