Was the robber moving? The coroner says NO. Pooled blood from the head wound and related evidence no doubt also showed that there had been no significant movement, otherwise the defense would have mentioned this attribute.
i haven't read anything in depth about it as you seem to have. the video doesn't show a whole lot. if he did indeed execute the scumbag when he wasn't a threat, that obviously is a violation of the law and he should be convicted.
at the same time, the defense can bring in their own experts to say the pool of blood can be interpreted different ways. no smears in the blood around his head doesn't mean his hand wasn't raised with a gun in it. there's just too many factors to consider.
this is just a cursory observation, i haven't done any serious reading into the trial transcript, so i can't tell you what was said or done.
what was the definitive proof that led to his conviction anyway?