If they had taken the opportunity to shoot him beforehand, according to the video I saw, it would have an execution. Also he's on record as saying he will never shoot first. And he never did. Additionally, with all the armed cops available, why would a taser cop break cover to a known armed man in flight? I don't understand why cops put themselves into harms way then use that as a lame excuse to kill. And, worse, Monday morning QB's defend it.
For the same reason that Monday Morning Quarterbacks find fault?