If an attacker is using a deadly weapon in the commission of a crime, that seems to indicate that the victim's life really is in danger continuously, as long as the attacker is armed.

Not necessarily.

AFAIK the common case where your logic falls apart is when an attacker takes flight. At that point, it's really hard to argue (under the law of every country I've ever lived in, including the US) that your life is in danger.

If they run away while still holding the weapon in a ready position, you could reasonably argue that, and there were cases when people did, and convinced the jury.

And I think it's reasonable - why should you try to guess whether it's retreat or merely changing position? And even if it's retreat, it doesn't mean that they won't shoot back as they do so (indeed, "fighting retreat" is a military term). The whole point of firearms is that they can do harm at a considerable range, after all. If the attacker wants to make their intention to stop being a threat clear, they can drop the weapon. If they don't, I'd rather err on the side of not second-guessing the victim.

"In war, the victors write the history." I can see how this would apply to the above situation.

Yes, this is why I'm saying it's the "common" failure case, rather than the absolute failure case.

To illustrate, there are very few cases in US law where you can shoot someone in the back.

>To illustrate, there are very few cases in US law where you can shoot someone in the back.

Unless you're a police officer. Then it's generally OK, and at worst you'll get suspended or maybe fired.