It’s simply because of the abusers in the system, just like with other professions like teaching, etc..
For example, the hate Blacks have for cops didn’t come from nowhere, and it’s not all about Black behavior. Anyway, a lot of people besides Blacks – including many Whites – hate cops too, and it’s usually for similar reasons.
There’s always the squeaky clean Fox News all-American image of cops that so many Richard Nixon types buy into. That image is often true, but it’s also often not true. The same people that would be attracted to White Nationalism are indeed attracted to law enforcement. What better place aside from militias would you expect paranoid, persecuted/delusional gun owners?
We need to also look at the fact that criminals, especially lower-IQ ones, are simply diving cops nuts. However, did the chicken come before the egg? What I mean is were the cops nuts before they became cops or were they made that way by the nature of their work and the types of people they tend to run into? There’s always a way – properly – to deal with obnoxious crime-prone people without going the Cluster-B way.