Being a cop is a tough job. I know, I was one for 7 years. But nothing can excuse the unprofessionalism and failure to follow procedures that I saw on that video tape.
Even the back seat passenger was yanked out and beat and for all the police knew he could have been an unwilling participant trapped in a bad situation against his will.
With as much as Felony Stop Procedures and Escalation of Force are practiced, there is no excuse for this type of behavior (I don't care what the circumstance are) from the people that we task with policing our society.