I find it hard to believe that a robber would shoot someone IN THE BACK after failing to rob the person. How often does that happen? Got any stats to back that up?
Really? The robber, who has no intention of actually shooting anyone, holds the gun to the guy's back and says "give me your wallet." The guy sasses back, the robber gets ticked off, loses it, and pops off a few rounds - unless the gun was a single-action revolver with a heavy trigger, it's very easy for it to get away from you - and then suddenly realizes that he just stepped into the big leagues; he's not a robber - a mere thief - anymore, he's now a murderer. The adrenaline pumps and he takes off for the hills. It would take more ice-water in your veins to stop and pick over the still-warm corpse than to just take off in panic.
Pretty easy, and pretty common.