Where did the perception of US Cops being "the bad guy" come from?

so as not to hijack another question posed here:

the responses to me were surprising. and in other questions posed and conversations that I hear; it seems that here in the US so many people have this prejudice against police officers. This perception of them being “out to get you”. Why? If you are living your life within the law, there is no reason to feel threatened by a police officer… no?