Seems like overkill even if he had been guilty.
There isn't exactly any one thing which goes wrong for the couple in this video. What happens is a whole series of mistakes made on the part of the police which ends up with a man being wrongfully arrested. To top it off, the arrest involved the car's owner having confusing and contradicting instructions shouted at him while at gunpoint. And all of this because the cops simply didn't understand how license plates on classic cars work.
The couple is suing the police department over the incident, which seems like a pretty reasonable response to us.