you can cite all the stanley milgram, stanford prison, psych 101 studies you want, but I don't think that gives a full explanation of the contemporary cop's psychology. the world we live in is one where the researcher is telling the participant, "DON'T TOUCH THE DIAL, YOU'RE KILLING AN INNOCENT PERSON," and the participant cranks it up to 11.
What even is the mind of a person who thinks that way? How do you explain tens of thousands of those people acting in concert?