This any surprise? Dog walking around wagging it's tail, shoot it.
Young black kid half your size talking shit in the jail as you walk him to his cell? Never mind that you have all the authority, better slam him face first into a cement wall and break his teeth.
Young teen girl acting bitchy? Grab her and slam her into her cell, then punch her several times, leaving her unconscious. Then, say she kicked you in the shin, after you banged into the toilet as you slammed your fist into her head.
60 year old woman on the phone with Onstar to get service for her broken down car? Interrupt her call and demand to see her license, then when she points out that she's done nothing wrong, curse at her and yank her out of the car. Then talk and get your lies straight with your partner so you don't get in trouble (only they didn't realize the women dropped her phone when they began to manhandle her, and Onstar recorded the whole thing, including them talking about getting their stories straight)
I think the biggest thing, is that most cops are cowards. They like to act like billy badass when they hide behind their badge, or are dealing with people that still have respect or fear for them. But, the minute that illusion of power and control gets threatened, is ignored by someone, they go off the deep end, and panic. They can't stand for people to question them, to doubt their place of supreme authority.
Of course, there are good cops, who can just sit there calmly and assess situations with a mentality that didn't stop it's maturation at the High School jock/bully level.
Thing is, even those "good cops" will close their mouths and back up their bad blue brothers for doing something wrong. All the incidents above, only reason anything came of it was they happened to be recorded. In each, either the surrounding officers went along with it, or just clammed up, and certainly didn't report it.