The police job attracts most people who do not have a college degree, are white, and lean into conservative/justice-heavy politics. Many of those people are bullies. That doesn't mean anything about people that do not have a college degree, or white people, or conservative/justice-heavy politics, it just means something about the police job and the people who agree to take it. We're looking at probability of (white AND lower college education) given (bully AND police officer), not probability of (bully) given (white AND low college education).
The closer you get down to beat-cop-that-interacts-with-citizens, the less likely you are to find a four-year degree or "conventionally liberal" beliefs. (Not that a college education or liberal beliefs confer any useful predictors about common sense, wisdom, ethical beliefs, or honesty.) This implies that anyone with a four-year degree and conventionally liberal beliefs is going to have a hard time understanding or influencing police officers by using any tactics that work on other liberal college graduates.
I agree with you that there are bullies from all political persuasions and education levels, but a very specific type of aggressive personality self-selects into the police job at a higher rate than in other jobs that require a similar background. Additionally, even if 99% of police officers in the US weren't aggressive and weren't bullies, that still leaves 10,000 "bad cops". It doesn't take many "bad cops" to make people angry and uncomfortable with the police in general.
In case my original message wasn't clear, I'm actually criticizing U.S. supporters of police reform for wanting police work to be perfect and ethical without actually volunteering to become police themselves. These (mostly left-leaners) entrust the enforcement of civil society to a group of people who mostly don't respect or agree with them, which is ineffective and hypocritical.