Or become that way after years of dealing with the best that society has to offer.
Police are people so there is no doubt that there are good ones and bad ones. There are some that are having a bad day because of stuff completely unrelated to the job just like everyone else. Except when it happens with them they aren't rude in some business meeting or surly in a job interview or take out frustration by giving someone a one week ban here. It doesn't excuse it but can help to explain it.
I've never been in law enforcement but grew up with it all around me and have at least one cop at every family function. I did spend a significant amount of time in environments where day to day experiences could have life or death implications. It wears you out. Not every Iraqi was planting ied's or launching rockets. But you know they are out there. And it is very difficult to separate the ones who would do you harm from the ones who just want to live their life...no different than you. Nothing pissed me off more than when some of our(America's) own guys did stupid **** that reflected poorly on the rest of us. The Abu Ghraib's of the world and things similar. I know through some limited anectdotal evidence that many cops feel similarly.(http://www.washingtonpost.com/opinions/think-walter-scotts-death-is-another-ferguson-cops-dont/2015/04/10/2f989444-dfa8-11e4-a500-1c5bb1d8ff6a_story.html)
I get the tenor of this board here these days. Overwhelmingly "bad cops", "power hungry cops", "blue wall of silence", and my favorite are the folks that liken law enforcement here to Nazis(that's an impressive display of intelligence). And there is no way to defend the indefensible(despite the best efforts of some of our antagonists here). It happens that "good people" come in contact with a "bad cop" and those cops should be punished accordingly. Personal opinion is that far more often that "good cops" come in contact with "bad people" and that can take its toll over the course of a week, months, years. I have no idea why any decent person would want the job.