Had he simply shot the robber 20 times, it would be possible to argue he couldn't stop firing - this happens even to LEOs.
But shooting the attackr, leaving, then coming back to shoot him again is not self-defense.
That's it in a nutshell.
I don't think any of us is arguing that the dead robber was not a scumbag. The question is whether Ersland's actions constitute the legitimate use of deadly force for self defense. His first shot -- absolutely. And, as Micro said, if he had emptied his gun into the perp
at that point it would likely still have been justified.
But he didn't. He left the wounded (or dead) robber, he
left the building, and he then returned to the building (went from a place of safety to return to proximity to the potential threat), walked right by the perp with one gun in his hand, went behind the counter (back to a place of comparative safety), switched guns, then came back to the immediate proximity to the wounded (or dead) perp (once again, went from a place of comparative safety TOWARD proximity to the threat) and emptied the gun into him, executioner style.
Understandable? Perhaps. Lawful self defense? Not a chance.