too many of those police are themselves white supremacists. Part of what's been happening here has been the infiltration (and recruitment) of white supremacists into the police unions, coupled with the back door money from conservative leaning donors and corporations and the militarization of police departments in the wake of 9/11.
Keep in mind as well that police officers are not that well paid, despite the fact that the pool of available talent is fairly limited. Many are ex-military. It is also a field where people with psychopathic tendencies can thrive, especially when protected by those same unions. I'm a big fan of unions, but I also recognize that one of the flaws that they have is that unqualified members are often protected from reprisal.
This is also not discounting the atmosphere that Trump and company are deliberately creating, hoping to push a narrative that non-whites are the enemy. It's propaganda, and as events in Minnesota this week prove, it's working.