In addition to the racist rogue cops (and I agree that they're a small minority--but even if as Trump says 99% are good cops, that means that there are 8,000 bad cops out there), there is a much larger number of officers who want to shield these bad apples. I can't believe that these defenders joined the police force in order to protect the murderers of innocent black and brown civilians. I understand the logic: "How can we do our job when our fellow officers are fired (or prosecuted) for ____?" But I think that's a false dilemma. Everyone benefits from punishing rogue cops.
Something happened to whatever civic or unselfish ideals these non-rogue cops had when they signed on, so that defending a particular vision of the institution became more important than "serve and protect." This is perhaps why they feel they have to portray themselves as victims.
My guess is that they are in fact victims, but victims of institution-think, which promotes the idea that the institution is more important than the people it is designed to serve. Most institutions do this, but most don't equip their employees with lethal weapons.
It doesn't help that Machiavellian politicians manipulate this sense of victimhood for their own benefit.