Here Nietzsche isn't discussing cops specifically, but what he says applies to them. Here he's actually talking about debt collectors -- also scum of the earth. I do not agree with everything Nietzsche wrote -- a lot of it is misogynist and odious in other ways -- but, damn, just like HL Mencken, when the man is right, he is damn right:
"[A] creditor is given a kind of pleasure as repayment and compensation—the pleasure of being allowed to discharge his power on a powerless person without having to think about it, the delight in 'de fair le mal pour le plaisir de le faire' [doing wrong for the pleasure of doing it], the enjoyment of violation. This enjoyment is more highly prized the lower and baser the debtor stands in the social order, and it can easily seem to the creditor a delicious mouthful, even a foretaste of a higher rank. By means of the 'punishment' of the debtor, the creditor participates in a right belonging to the masters. Finally he himself for once comes to the lofty feeling of despising a being as someone 'below himself,' as someone he is entitled to mistreat—or at least, in the event that the real force of punishment, of inflicting punishment, has already been transferred to the 'authorities,' the feeling of seeing the debtor despised and mistreated. The compensation thus consist of a permission for and right to cruelty."
--Nietzsche, "Genealogy of Morals," 2nd essay, part 5
Bingo!
-B.