I take exception to the term 'execution shot'. Can you prove that the armed robber was not dead yet or that he wasn't still moving? Can you prove that it was an 'execution shot'? How many times have we seen police shootings where the perp was shot a number of times but still lived?
So, if he was still moving and alive he still posed a potential threat. If he was already deceased the act of killing was done in the initial string of shots and that final shot could not be murder simply because you can't kill an already dead person. So in either case, the first, he still posed a threat. In the latter, it would not be murder, but rather a different crime such as desecrating a corps.
I posted this because this is pretty much what every juror on the Grand Jury would hear and also what they would hear on a regular jury as well. So considering the totality of the circumstances, is it worth dragging this man through the legal system over a misdemeanor?