This is NOT some kind of chivalrous exchange between identifiable enemy combatants. And if it were, it would be a mutually agreed upon code of conduct adhered to by both sides.
That is clearly NOT a relevant consideration in this instance.
It is relevant for whoever is passing judgement, or in said situation.
And in this situation, I think if I've already given him my stuff and he's walking away, I'd call the cops, get a good description and direction of travel, and let them handle it.
Even if you're legally allowed to shoot someone in the back from making off with your stuff, and even though it probably is a lowlife, there could be unseen results of taking such an action. Distress over taking someone's life is something most could do without. And even if you are justified, it could go to a trial where untold thousands would be spent to prove you were correct.