What if you fire 4 round of .45ACP in quick succession? The second one would likely be the kill shot, but you fire two more after that?
I am not a Doctor, but an observer in human behavior. When I observe that his threatening behavior stops, I stop responding to his threatening behavior.
There's probably a reasonable expectation that you not be firing with your eyes closed. If prior to the third shot you don't see that the attack has stopped, then you're probably okay.
I would say that around the fourth shot there should probably be some consideration of just how deadly what remains of the attack is. If the attacker is still on his feet and still has a gun in his hand, I would say you're on safe ground.
The defensive handgun class I took was based on pretty well-accepted practices. They taught that one fires two shots to center of mass and then checks for whether there is still a threat. The third round, if one is necessary, is a head shot, based on the possibility that the target is wearing body armor of some kind.
The training is not based on giving the attacker a chance to survive. It is based on making most efficient use of resources in stopping the attack. After the second shot, one is expected to reposition themselves if possible as a defensive measure.
One should also consider the distinct possibility that there is a second or third attacker. It wouldn't be prudent to waste four rounds on the first attacker and then have insufficient ammo to stop additional attackers.
Firing four shots without consideration of what the threat is doing increases the chances that the attacker has turned to flee and a round might catch him in the back. It's not the end of the world, but it might take some explaining later.
If the perp was still moving forward, he was still a threat. The Sherriff's deputy who taught my CC class said to fire as many rounds as necessary to "end the threat", but not to "kill the bastard".
He then, in a round-about way, said "If the bastard wasn't dead, the threat wasn't ended"...