It is not so simple. I would agree in large extent to the notion that if the threat to one's self is determined to have passed, an end to the hostilities on the part of the good guy should cease. However, circumstances do arise where, although the immediate threat to the good guy is over, there still exists a threat to others. I cannot explain it fully enough on my own, but if the BG is running away, but still poses a threat to others, continued use of lethal force may still be justified. Remember, use of Lethal Force is not exclusive to the individual and his/her family, but extends to third parties as well, even strangers. Enforcement of the law is not exclusive to local, county, and state police. Authority is granted to them, but the responsibility is inate in all of us Citizens.
That pretty much say's it all. You are justified in stopping the threat, but once the threat has ceased, you are required to stop as well. In the case of the BG who turns and runs out your door when he see's you're armed, you must stop firing when he turns and leaves. If you continue (you go to the door and shoot him as he's running across your front yard) and kill/injure him after the threat to you has passed, you will be held responsible and, probably, arrested for manslaughter or attempted murder. You might run into a grey area if the BG has already comitted a violent act like shooting at you, since you could say you thought he was only retreating to a safer spot to fire again, but that's something you'd have to decide at the time.