Some of the police brutality comes down to poor decision-making due to poor training and little experience. I never saw it that way until I read Malcolm Gladwell's Blink. There's a chapter where he discusses an incident in New York where cops shot up an innocent guy. The analysis was that in high-pressure situations, the mind ceases to function and focuses purely on survival and sustaining one's self. One becomes essentially autistic in being unable to recognize social signs such as facial expressions, body movements, etc, and the outcomes when weapons are in play are often gruesome.
It was quite an enlightening read, and I think I take it seriously because my own experience in high pressure situations. First was a time when I was taking a walk with my dad and some kids surrounded us. One tried to spray pepper spray in my face from behind, but my dad saw he was doing something so yelled at him, and my dad got sprayed instead. Then they all ran. I was stunned and couldn't process what was going on; really, I was just so focused on the guy in front of me, my decision-making process was paralyzed.
My second was when I was the venue technology manager for Cypress Mountain at the Vancouver Winter Olympics (freestyle skiing and snowboarding). In terms of operational pressure, that was the most difficult thing I ever experienced, and even more so because Cypress was literally the most difficult and complicated venue of the Olympics. There were situations where my brain literally just shut down and I was going 100 miles an hour trying to fix whatever went wrong; there's a word for that, it's called panic. If it weren't for the training I had and some of my more experienced colleagues, I imagine it would have been an utter disaster.
I can only imagine that if I have some guns, batons, or any other type of weapon, and am meant to quell a situation, it could get ugly easily.
edit: One more anecdote, my friend is an immigration officer. He had an incident once where he and another officer were interrogating a guy because some papers or something needed clarification. The guy suddenly made a move, they misinterpreted, and it became an all-out brawl. My friend distinctly remembers yelling at his colleague and his colleague yelling back at him, but they could not hear each other. Their minds were in a zone where they could no longer communicate, they were just subconsciously focused on survival. Later, it all turned out to be a mistake, but in the heat of the moment, it was hard to discern that.