Power is corrupting, and police have a
de facto shit-ton of real, immediate power over everyone in their vicinity. They have a "martial law bubble" around them that allows them to physically assault you, put you in jail, and upend your whole life with very little effort. You will have no recourse, and they will suffer no consequences. You may file a complaint with the department, and it will be followed up on, poorly, by a fellow officer who is highly motivated to see nothing but another disgruntled loser trying to get petty revenge. The judges don't care and work with the DA and the same officers all the time, and maintain cordial, cooperative relationships with them, all of which is damaged if there is any adversarial action.
It is within this context an average intelligence, perhaps even above-average moral person enters after 6-weeks of training. This person likes watching football and drinking beer. They help their elderly neighbors with the trash. They are a "normal" man who will stand up for himself, physically, in extreme cases. Now give him a gun and a badge and a martial law bubble. How do you think this normal, average man will evolve when he realizes his power? When he sees his friends on the force us it, daily, to get what they want, to make their jobs a little easier? When does he learn to enjoy that feeling of power, for itself, the way it feels to beat someone and put his knee on someone's neck because they talked back to you, and know that no-one can do shit about it? And you can even rationalize it - they shouldn't have been rude, they should know how much power I have and that I'm only human, its really just punishment for being stupid. Plus, the public still loves me and is grateful to me, considering me in high esteem as a moral upstanding citizen - so we must be doing something right!