How are you to know someone is truly incapacitated? That requires that a civilian, under attack have special ability to determine you are no longer a threat. Also requires that same civilian, who may be freaking completely out of their mind, act with the professionalism of law enforcement... ridiculous.
If he’s not moving, he’s not a threat. How subtle can that be?
As I said, you can kill him if he moves, but he must move. If he doesn’t move, you’re not likely going to win with either the judge or the jury.
Also, nobody is expecting you to take a pulse, or pupil dilation.
Do you really need to be an expert to decide if he’s moving or not, if he sits up?