Though it might sometimes seem so especially on social media, in reality not all of us just blindly hate a person because they belong to a certain group.
It's literally the job. I really don't care if you think you're doing the right thing while oppressing addicts, homeless people, sex workers, etc. They could get a job that actually reduces crime instead of punishing the disenfranchised but they don't.
There are probably many many people that are police that are trying to do good. But police in America have a huge PR problem. Doesn't help that there are departments that don't hire quality people for the job and that there is a union that is a bit too aggressive in there defense of some police that probably do need to be fired.
My feeling is that the police in general are under trained for the work that they are asked to do and in turn everything problem is a nail that needs to be hammered down. It breeds an "US" vs. "THEM" culture in the departments.