I don't know how anybody can defend this one.
I think cops who pull that strict questioning bullshit are the most dangerous. This cop was so good at it that his tone didn't crack until "again, the front license plate".
Few things I noticed right off the bat:
The cop got a name that matched his records on the vehicle.
That gin you guys talked about was not unopened, nor was it gin.
Sam says something about going home (my assumption is to go get his ID - it's hard to hear him at times) after being repeatedly asked about the ID, and not long before he starts the car.
So, you got a front license plate offender who seems to be attempting to flee. You know who owns the car. The driver verified who owns it and claims to be her husband who lives in the neighborhood. Assuming they live together, the car registration address would likely have been nearby.
It appears the cop forgot all of this as soon as Sam turned the car back on, which I'm sure spiked his adrenaline. While I can sympathize with the difficulties in being a cop, nobody can excuse these actions in these conditions.
There was no indication that he was aggressive or about to commit some crime, violent or otherwise. A low speed chase/follow would be the proper protocol most likely. Nothing more. Certainly not deadly force. That's just fucking retarded.