I don't agree with your ultimate conclusion, but I'm surprised to find myself agreeing with your basic orientation towards the event, which is a marvelous example of karma in action.
--The couple shouldn't have parked in that spot if there wasn't a handicapped person in the car
--The white guy with the gun should have posted himself, vigilante-style, in the parking lot and insisted that he was the patrolman with the right to hector the scofflaws who parked illegally (and it is illegal) in handicapped spots
--The (black) husband, seeing the white vigilante adjacent to his car and speaking in agitated tones to his wife, shouldn't have come up and knocked the white vigilante down. That is felony assault.
--The black husband only backed away when the white vigilante pulled out his gun and aimed it at him. That's not "backing away," regardless of what the wife (and later black crowds) insists.
--Yes, the stand-your-ground statute was very likely in effect. But the white vigilante certainly didn't need to shoot the black guy. The black guy was backing away. On the other hand, the black guy might well have had a gun himself--and given the extremely aggressive way in which he knocked the white guy down, you can't blame the white guy for not wanting to find out whether the black guy had a gun.