Is he in fact, immobilized? Or was it just testimony to that effect?
The coroner testified that the bullet fragment in the brain had rendered the robber dormant, likely to survive, but with issues.
As for first degree murder, Ersland had to go over to the drawer of his desk and unlock it in order to retrieve the KelTec that he used to kill the robber.
It wasn't as if he felt he was in danger, fumbling to get the KT out of the drawer, then firing from a position of cover. Nope, he just walked right over and shot him multiple times and walked away as casually.
Of course, then there was all the bragging about how he had been shot in the robbery (no evidence the robbers ever fired), that he was a war hero (which he wasn't), and so on. Heck, his testimony of how the events went down didn't even come close to matching the video.