huntinaz and chadstrickland, Ersland ended the fight when he rendered the first robber unconscious. Nobody has a problem with that portion of it.
You can keep making up all the hypothetical stories you want; you can paint as ugly a future for the decedent, had he lived, as you like; you can argue your point until you're blue in the face.
You may believe in An Eye for an Eye. If you really think society would be improved by returning to the days of Hammurabi's Code, you need to review your ancient history a bit more closely. We haven't had Eye for an Eye since the days of English Common Law or the Napoleonic Code, neither of which recognize the concept; and most of us think that's an improvement.
The fact of the matter is, Ersland had ended the fight. Period. The dead kid was not dead yet, but unconscious and helpless. Ersland chose to retrieve a second firearm, and pump five bullets into him as he lay there on the floor. For an attempted armed robbery, and a botched one, at that.
If the guy had killed Ersland's pharmacy tech, I might see a lesser charge; if Ersland had been seriously injured, I might see a lesser charge. There still should have been a charge, but I'd have been more able to accept his actions as having been caused by fear, or rage, and beyond his ability to control.
But that wasn't the case, and what Ersland did was first degree murder. (Instead of second degree murder or manslaughter, which I'd have seen as reasonable in the other hypotheticals.)
You don't retrieve a gun, walk over, and shoot an unconscious person on the ground. A US soldier might be able to make the case that he thought the guy was playing dead, and lying on a live grenade, but even then that soldier would be under scrutiny - and that's in a combat zone.