Ersland chases the second man outside, then goes back inside, walks behind the counter with his back to Parker, gets a second handgun and opens fire.
This, personally, is what clinches it for me. If you feel safe enough to turn your back on a man, you are not currently "threatened" by him. The "kill or be killed" mindset must have already subsided, or at least it should have. If Ersland had peaked his head around the door and fired a few more shots from there, I would have been more sympathetic. I might have even said that was in self defense. However, specifically ignoring a person, then coming back to shoot them reeks of foul play.
Imagine an old WWII movie. An American is shot, and lying on the ground, gasping for breath. A Nazi commander walks into the room, surveys his men to see that they are in position, then takes out his pistol and shoots the American in the head. Seems like a typical scene from a movie, just to show how evil the Nazis are. Heck, Lord of the Rings does a similar move in the third movie with the orc commander shoving his spear into a downed human soldier. Simply put, you don't hit a man while he's down, and you certainly don't shoot him.
That said, I think Ersland's initial reactions were reasonable and might go as far as to say commendable. I find it perfectly acceptable to kill a person who is threatening to kill others (just threatening you is a slightly different matter for me). In addition, I believe that first-degree murder is the wrong charge, and that second-degree would be better. However, I'm also not a law professor, so perhaps the difference is too fine for me to see. Nevertheless, the execution probably didn't matter in the long run, as Parker probably would have died from the first shot.