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Many of these close questions of fact are ultimately jury questions for a jury to decide.
There are cases of a robber taking your wallet by force then he takes a few steps away and shoots you.
As rotor pointed out robbers are unpredictable.
Distance is certainly a factor. If the robber is 500 yards away and you tried a "sniper" shot then that would be more problematic than if he was five feet away with his back turned.
I would think District Attorneys have to take a very close look at the facts then decide whether to present the case to a grand jury and ultimately to a petit jury.
Juries also disagree among themselves which is why you see mistrials on occasion.