This isn't even a cop thing, though I have no doubt the rate would be higher if empirically tested; cops tend to be very Conventional Morality in the sense that If Legal -> Moral.

https://en.wikipedia.org/wiki/Milgram_experiment

TL;DR: 65% of people would kill someone if given an order to do so (and some plausible deniability).

My money's on it being 70-80% for US cops, maybe higher. My only real metric on this is the one guy I knew who made an arrest he was ordered to that he (should have) known wasn't legal and shrugged his shoulders about retelling the incident, saying "I'm a soldier (he was an Army veteran); I follow orders."