I have active Ranger as an instructor. We talk and go thru a number of scanarios regarding what to do in this or that situation. Clearly,you rarely have a confrontation needing force.
I was also taught that each round you fire probably carries a minimum price tag of ten thousand dollars.
I'm less concerned about being too bold than I am about being too cautious. Many times there are very few seconds available for decision-making.
The poster claiming that one cannot shoot an attacker in the back is over-simplifying, I think. Just because an attacker shows you his back does not guarantee that he is no longer a serious threat.
Did the shooter at the Waffle House shoot the robber in the back? Perhaps. I see no ethical problem with not issuing a warning to an armed robber threatening the lives of other people.
Similarly, the mere fact that an attacker is unarmed does not mean he is not a serious threat. I'm of average height and weight, 67, and pretty much recovered from some serious medical problems. I'm not going to take a beating that could kill me just because the attacker is "unarmed".