You wouldn’t get a fair trial.
The presumption is is that the police are credible and moral actors who perform their duties in good faith.
It doesn’t even matter when it’s caught on video. I remember a few years ago the first time I saw video footage of a policeman shooting an unarmed man from point-blank range. As far as I know, the cop wasn’t charged at all.
Since then, I’ve seen dozens of videos of police shooting and beating to death people who pose little to no immediate mortal threat. I was also acquainted with someone online who shot multiple times and paralysed by cops (as far as I know, he successfully sued) and who later died from his resulting ill-health and injuries a year or so later.
In almost every case, the cop gets to claim self-defence and the case rarely ever goes to criminal trial.
The opposite is practically unheard of. I don’t think there’s any precedent of anyone successfully pleading self-defence in the killing of a cop.
I can imagine some scenarios where you may succeed in a self-defence plea. The key is to have a higher social status than a typical police man—like a CEO, rich banker, or a
high-profile public official. The difference is is that the police protect those people, rather than threaten them with death.