The problem is the pool of humans from which cops are drawn: trade school burnouts, fucked up vets, 2nd amendment gun nuts, right wingers, high school drop outs, power hungry beta males, people too stupid to get an advanced degree or learn a trade.
Honestly I think we should just send all cops in the US to death camps and start from scratch with a force of like 1/8th the current size. There would invariably be some "good one" caught in the mix, but it would be for the best.