After living in Europe for six years now, my wife is still puzzled sometimes by the differences between Europe and North-America when it comes to the police: how they are experienced by the population and how they see and present themselves and which role they think they're playing in society. Big difference. I'm certainly over-generalizing but here, we see cops as approachable and helpful in general (with exceptions) while in North-America, at least my wife's impression is that of cops being mostly intimidating (again, with exceptions).
Of course, this is all complex and different social and societal aspects play a big role, such as e.g., the odds for a cop of running into an armed person. But when I read how the police handled the situation with the group of black trick-or-treaters, it seems so foreign to me now from a more European perspective.
I suppose accountability is always going to be an issue - who watches the watchmen? But it should not be - in a democracy especially, there should be functioning mechanism to prevent abuse of power, and that of course applies to police actions, too.
The US has always had this exclusionary divided culture due to its diversity and racism. It makes it hard to fix some of the larger issues that exist, since much of the debate is framed as "us vs them".
You can see similar issues in countries like Malaysia, where the parties are mostly split on ethnic lines.
This isn't excusing this behavior at all. Just context.