Robber had an airsoft M&P replica pistol, according to police. Customer who shot him appears to have a compact semi-auto 9mm. The customer was under no legal obligation to remain at the site of the shooting. Police want to ask him questions, undoubtedly as to whether or not he felt in danger of death or great bodily harm for himself or others. His attorney will surely advise him on what to say to police. Some have said that he shot the felon too many times. Down here in Brevard County, Florida, our Sheriff would say that "dead isn't dead enough for felons" like him. A trained LEO might catch some heat for a few extra shots, but not a civilian. Just before he passed the armed customer, the robber pointed the gun at him and the man sitting with him. That's well within the boundaries of being in reasonable fear of death or serious bodily harm. As long as the bad guy is moving, has the gun or within reach of the gun, one must assume he is not out of the fight yet.
I can almost hear the robber's family... "My boy dindu nuffin' to shoot him for. He's a good boy". That culture of violence is also a culture of denial.
Taking a life, even a despicable life, is no small thing. This will surely haunt the shooter and other customers who were victimized.