The law in such cases differs from state to state, from country to country, and "cold blooded" is certainly defined in a variety of ways. "Cold blooded" is certainly not the mental state of someone who has just been assaulted or robbed, so that's where your argument fails. What happened, what reaction has been provoked in the aftermath of the robbery attempt, is in my opinion, entirely the responsibility of the robber. That is where we differ. No robbery, no "cold bloded execution", simple.
That video is disturbing. I’m surprised that the M.E. determined that the perp would have “survived” the head shot. I wonder why the defense didn’t challenge that. If he was already dead (I’m sure you could find an expert witness to attest to that), then the bullets to the abdomen would have made no difference in the outcome.