My take on it is that "stopping the threat" is only seen in hindsight. That doesn't mean that we can defend extreme use of force by saying "how could I know that he was dead after the first shot?"
I don't believe that emptying a magazine is a smart thing to do at all, but shooting at the guy until you feel that the threat is over is expected.
When should you stop shooting? When the threat seems to be over.
Is the threat lying on the ground, out cold? Did you just center mass a load of buckshot? Rifle round to the chest? ..honestly, anyone who has done any learning on this subject will know when to stop shooting, and the shooting had better stop, or its not going to be a good thing.
Whatever the situation, capping an unconscious person in the head is inappropriate. Dumping an extra five rounds in his back.
you aredefending yourself, not deer hunting. don't just follow your instincts or desires, your anger, stay in control and only do what you have to do to remain safe.
At the very least, remember that every round you fire also presents a serious threat to anyone within the range of a stray bullet. Seeing a cop download a full magazine into the guy's processor is not an example to follow. You will be accountable for whatever happens, and you won't have a badge, free lawyers, and so forth. You're just going to be the guy who shot a teenager fifteen times, with ten of those rounds in the back.