The moment you are straddling someone punching down at them, you are the aggressor. Period. There is no set of circumstances short of wrestling for the cop’s gun (which she was obviously NOT doing) that would justify beating her about the head. He was not acting as an LEO at that moment, and he should be charged to the fullest extent of the law. If a cop can do that and not be charged, then there is no law at all in the US today...
Well that could be.
I’m not a regular watcher of law enforcement.
I could be totally wrong.