Here's a thought experiment. Say we have roving death squads deputized to kill bad guys at will. What happens when the death squad guy turns out to be a bad guy? How do you prevent the death squad guys from killing whoever they want? Or even each other? The best answer we've come up with so far is a fair trial.

If a man points a gun at someone, they are a threat. If a man is completely subdued and immobilized, face down on the ground, completely surrounded, they no longer pose a threat. Killing someone who does not pose a threat is murder.