I was trying to think what I would have done if I had witnessed the policeman killing George Floyd.
First off, I think recording the incident is mighty important. Because there's no expectation it's going to lead to death or permanent injuries, but it is a case of police abuse of a suspect. And people will likely not believe your word over a lying gang of cops, unless you have the video evidence of what happened and for how long and without resistance.
Secondly, it's hard to intervene when there is another cop specifically there to do crowd control. You'd have to physically push past or grapple with the goofy little cop to get to the perpetrator cop. Without him there, the chances of going right up to the perp cop and trying to get him to refrain would go up a good deal.
Third, it's really hard to believe that the police don't know what they are doing, and that they will actually kill a guy for no reason in broad daylight on the street while it's being witnessed and filmed. They are the police afterall.
4th, I think I'd be less likely to intervene if I was with a female friend, as some of the male witnesses seemed to be, as you don't want to put another person you know in harm's way.
5th, it's easy to imagine yourself handcuffed and prone on the ground with a knee on your neck for trying to help Floyd. And that wouldn't do nobody no good.
Otherwise, I think both aging and my time in China has dulled my will to intervene. Language and culture barrier means I might not know fully what's going on. Or be able to explain myself so well if required to. So I've become more of a head-down bystander than I would prefer. A few times I have asked a woman if she was okay when a guy on the sidewalk seemed to be manhandling her late at night.