Jim, your question really requires an answer of a few thousand words more than I'm allowed here in GATE, but I'll "give it a shot," so to speak...
You're absolutely correct that firing a gun at a human being is an act of deadly force. "Shoot to wound" implies that by your own lights, he didn't need killing, yet you did something to him that could have killed him. (Lots of folks have hemorrhaged to death from leg shots.) That's pretty much the same as admitting that, if he died, you used more force than even you thought was warranted.
"Shoot to kill" implies that your purpose is ending his life and taking him away from those who loved him. It smacks of malice. Malice is a key ingredient in a Murder charge.
This is why we teach "Shoot to stop." Political correctness has nothing to do with it. It goes to the heart of the matter: WHY YOU SHOT THIS MAN. You shot him to STOP him from doing horrible things to you or other people, things that would result in their death or crippling injury. THAT is a LAWFUL purpose. Hence, we shoot to stop.
You can, for example, justify an intentional shot to the pelvis if you can show that you knew most MDs say a broken pelvis will usually cause a man to fall within a step or so. He was charging you with that baseball bat you mentioned, and you knew that if you could inflict a wound that caused him to fall within a step, you could STOP him more effectively than with a bullet through the heart that could leave him up and running and clubbing people to death for more than ten seconds. You weren't "firing to wound," because the pelvic area is filled with large-bore arteries that can cause death by exsanguination very quickly when pierced, for one thing. You were shooting to stop. But, because fewer gunshot wounds to the pelvic area cause death than gunshot wounds to the heart, you obviously weren't shooting to kill, either.
We aim for center chest against a violent offender who is armed with a gun because, as a general rule, it neutralizes his ability to effectively shoot us or others sooner than anything else: given the predictable degradation of our own marksmanship under stress, it gives the highest hit probability. The pelvic shot would take away his mobility to stab or club us with a contact weapon, or take our own gun away with his bare hands and shoot us with it, but would not deny him the ability to fire his own gun effectively once he fell. The purpose, again, was not to kill him OR to merely "wound" him, but to STOP him, and the center chest aim is supported with great strength by a long history of police and armed citizen training.
Jim, I hope you find this of help.