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.
Careful now. That's some stretch you've got goin. You might sprain something.
Columbo would probably conclude that
you're the robber.... or that he's in your family.... since you seem so willing to help explain how it probably happened....lolol.
That's only in the movies though. Real life motives usually translate into an
agenda.