The law is quite right to punish cold blooded executions of unarmed individuals. Had Ersland been satisfied with a headshot, he’d be a free man. But he decided to do an evil thing, and he got caught.
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.