I suspect some cops are assholes*, but I suspect a lot of stupid monkey-brain thinking that cops are 'Us' and the protestors and residents of Ferguson are 'Them', which multiplies assholish behavior.

(A bit like whenever you find a crime in an organization; you only need one genuine criminal, and a bunch of otherwise-pleasant people who hinder attempts to deal with that because they have a case of the Stupids that insist they Protect the Tribe! Down with Invaders!.)

* On the basis that the set of cops in the Saint Louis area is large enough I would be shocked if there wasn't at least two assholes. I also suspect assholes want to be cops more than, say, computer programmers.