Everyone forgets, cops are people also. They are just as good or bad as the rest of us. Now, with that said, the cop should be fired, charged with unlawful detainment, assault with a deadly weapon and being a jackass, tried convicted and put away.
I seen the same thing happen way back while living in NYC. I was in a neighborhood store when a Transit cop came in. He got coffee and while behind me in line, some guy comes in and asks the cop how long he is going to be. The cop car was blocking him in. You would have thought the guy asked the cop if he could fuck his 10 yer old daughter. The guy tried to calm the situation down, but ended up on the ground with the Transit cop's knee on his neck. If the store owner and I didn't start yelling at the cop to back off, I believe this guy was in for a world of hurt, then a trip to jail. That cop was totally out of control.