While I was one to bring up Rodney King... And I am old enough to remember it... We know that King was a recidivist, and in all truth probably got off lightly that night as already mentioned. As to tactically correct handling of that incident, I can certainly understand that in the heat of the moment, even the most well trained LE can make tactical mistakes.
I have also witnessed inappropriate force being used against a suspect by police... It happens...I will even give the police the benefit of the doubt most times.
Sure, in big cities, and small, there are opportunities for LE to be corrupt. It's probably rarer today that it was years ago... but back when I was younger, some of the best parties were thrown by people in the criminal justice system (on the good side) with substances obtained in encouters with misdemeanor offenders and LEOs were in attendance and partaking.
Unfortunately for LE... any deviation from perfect behavior, makes them "dirty" "corrupt" or otherwise "bad."
And, Gasmitty... the King thing may have been "edited for prime time" but we all know that there are other exceptions to exemplary behavior that prove the rule... most times the bad ones get caught, eventually.
SO back to the OP... yep, it could happen... I would not be able to personally intervene without too great a risk... this is a job for the press... Most lkely I don't have a DA, or state cop's number on speed dial... But most of the news outlets around here have a "*9 or #222 on your ______cellular phone breaking news hotline" to call...