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.
...how could this be justifiable?
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.
What (some) european countries have is a much lower violent crime rate, which might jade perceptions either way.
Continuing the theme, many police actually take indirect bribes. Look up Patrolmen's Benevolent Association cards and the like. There are different levels of tokens depending on connectedness and donation level, and you can find them openly discussed on police forums. I would guess the only reason we haven't developed a culture of on the spot cash bribes is that police would take the money and write you a ticket anyway for having insulted them. They have a psychological need to pretend they are on the right side of the law.
Simple reason being, that random police violence just results, ultimately, in the kind of uproar and civil unrest the US experience now. Basically the last thing dictatorships want.
Obviously the ideal solution is to 10x the capacity of the criminal justice system but nobody's paying for that.
Outside of big cities it gets even more lax. You just see police officers hanging out like regular people. I once saw an old guy get into an argument with the cops that looked like an argument between two people, not between "officer and civilian". In the US that wouldn't happen, the officer would feel slighted and probably arrest that guy, or the guy would never dare to talk back to a police officer in the first place.
But to your point I suspect the military police is very brutal.
Perhaps less people would see fascist, totalitarian overtones in law enforcement if some segments of the police wouldn't openly state this.
Roma were literally being sent to death camps to be exterminated throughout much of Europe 75 years ago.