The guy was so distraught from being robbed that he was shaking and accidentally gripped the gun too tight and it inadvertently discharged. The fact that the robber was hit in the head was just pure bad luck.
Actually, something along those lines, maybe little more logical, would be a good defense. I'd simply get an expert to testify to something like PTSD, or extreme emotional disturbance, or heat of passion, to lessen the "intent" angle, and maybe get a manslaughter, criminally negligent homicide, or even an acquittal. It was a bad shoot in most jurisdictions, but that doesn't mean the guy deserves a murder conviction or prison time. He was the victim of a robbery after all. All facts must be considered, but there's no way to call it a good shoot, at least not legally.
