I know a few decent coppers but like every situation in life there's that balance between control and not wanting to be controlled. Often people who've experienced negative feelings of being undermined, controlled, not treated well - when presented with an opportunity for deviance they act upon it.
Take the example of the man who's been bullied (a cliche for coppers I know), or the man who's been rejected by all the tarts in town, the man who feels intimated by the dickheads, the gangs, the hench blokes, the bully types, the young lads who have nothing to lose so offer you out.....this is the control balance in action. You don't have much control and you resent it. Give these men the opportunity to do something that gives them control and often they will act upon it.
Add into the mix a narcissistic type personality where any rejection or questioning of the ego is met with resentment and you have the fuel for a classic dick head copper. Which unfortunately seems to describe at least 70-80 percent of the cunts.