We all know that people have been killed by a punch. No one is arguing that.
What I’m arguing about is what the law IS, not what the law should or shouldn’t be.
The legal standard for justified use of deadly force is whether you are in immediate danger of death or grave bodily injury. Rightly or wrongly, most courts will rule against you if you shoot someone who punches you in the head, with some notable exceptions.
Some of those exceptions - disparity of force. As a near senior citizen, if George St Pierre squared up against me and said he was going to kill me, the court might well find me justified in shooting him. In the Trayvon Martin shooting, Martin had mounted his victim and was smashing the victim’s head onto the concrete sidewalk. The court rightly found the shooter not guilty.
But in most cases (not all, most), you’ll have a hard time in court if you shoot an unarmed man who punched you in the head. I’m not saying that is right or wrong, I’m saying that is what the law is.
Furthermore, for deadly force to be legally justified (note, I said LEGALLY justified, not MORALLY justified), the threat has to be immediate. If the attacker stops and walks away, legally the threat has stopped. If you use deadly force after the threat has stopped you are likely going to be tried and convicted of murder.