No matter how anyone responds to this video... No one has all of the facts.
From what is presented... A guy got robbed and when the cops finally got around to helping him they responded to an obviously upset individual with a beat down.
I know exactly what I'd do in this situation because I'm not the everyday average joe and have deployed to war. I have absolutely zero tolerance for people who abuse their power and being raised properly I know that being granted power doesn't mean I have the right to abuse the people that I have power over. It means I do what I can to help them and if someone decides to attack me I have every right to defend my life.
That officer had absolutely zero justification for what he did no matter how you try and rationalize it. The ONLY reasons you have for backing up that cop is because you blindly follow a badge and/or you have forgotten the most basic principals our Constitutional Republic were founded upon.