Legal matters are one thing. Are you legally justified in using what would be termed 'lethal' tactics? Then it probably doesn't matter whether you dispatched the bad guy with a single head shot, it was legal. You should have the protection of having used 'legally justified lethal force'.
Does that jury, civil or criminal, have any concerns about the fine points of a case, civil or criminal?
Imagine a different scenario, a silly, nearly ridiculous one, but relevant, nonetheless.
You have been maced. Guy had a bottle that he said was kerosene, a lighter, and he has vowed to burn you alive to kill the demons inside of you.
You are blinded, and have nothing but a 7" square head screwdriver, and since you legally, legitimately fear for your life, you start stabbing and don't stop until your arm is so numb that you drop the weapon.
Really, what should the system think? Here's a guy whose entire body is little more than a bloody sponge, you did it, and if there are problems with your statement, wth?
At some point he was no longer a threat but you went on for ten minutes and killed him. At some point he was dead, so you desecrated his corpse for another ten minutes.
What will the entire civil and criminal system think and do?
Here we are, saying that we should without giving it even a second of consideration, put two bullets center chest, and then one to the brain? Am I supposed to shoot until I hit the head, do I keep shooting until I'm sure that I got his brain? How long should I wait until he is so still that I can be certain that I will hit his head? In my case, I'm going to have to get within ten feet, put my glasses on, then get out my flashlight
Seriously think. These statutes generally only authorize use of lethal force, but don't give carte Blanche to ensure that the guy is as dead as if he was hit by a truck. Every shooting is a special thing and every legal case becomes a unique challenge. Whether it's putting a couple extra bullets into a guy to ensure that he's not going to shoot back, or running him over with a truck, the questions will be asked by prosecution whether you were justified in shooting, and the civil courts won't have that same burden of justification.
A tattoo that says 'two to the chest and one to the head, keep on shooting until he is dead' isn't going to help.