On the Genealogy of Morals:

The debtor made a contract with the creditor and pledged that if he should fail to repay he would substitute something else that he “possessed,” something he had control over; for example, his body, his wife, his freedom…

Let us be clear as to the logic of this form of compensation: it is strange enough. An equivalence is provided by the creditor’s receiving, in place of a literal compensation for an injury (thus in place of money, land, possessions of any kind), a recompense in the form of a kind of pleasure—the pleasure of being allowed to vent his power freely upon one who is powerless, the voluptuous pleasure “de faire le mal pour le plaisir de le faire [of doing evil for the pleasure of doing it],” the enjoyment of violation….In “punishing” the debtor, the creditor participates in a right of the masters….The compensation, then, consists in a warrant for and title to cruelty.