Try this scenario:

You are armed. You are walking down a dark alley and a man steps out of the shadows clearly holding a knife, and the man says, "Give me your wallet." No one else is in the alley. It's just the two of you. The only exits are behind the man holding the knife or back the way you came.

In this scenario, a reasonable duty to retreat could be seen as you taking a step back and saying that you are armed and will shoot him. He takes a step towards you still brandishing the knife, you draw your gun and shoot him and he falls to the ground. He's still alive, but is wounded and is no longer a threat to your bodily harm.

Your duty to retreat continues. You can't just keep shooting the man. Now that he's no longer a threat, you must stop shooting him.