Law enforcement in the states wouldn't go that route. Basically, in a valid self defense situation, you shoot until there is no chance of the bad guy posing any further threat. You are fully within your rights to keep shooting until they're done moving, because if they are not completely incapacitated they can still kill you/others. The only rub here would be that, depending on the state, they may try and argue the bystander had a duty to flee rather than using force. That said, he could fairly easily argue that he had no safe means to flee, so he'd be fine as long as he took the appropriate actions after this incident.