What are the motifs in The 120 Days of Sodom and Other Writings by Marquis de Sade?

Virtue vs. vice is a recurring idea. Sade's tales are full of characters exhibiting the most virtuous of traits as well as the most debauched and vicious imaginable. He frequently makes a point of contrasting these types of characters, often depicting the virtuous being victimized by the the vicious and vice winning out. In Sade's response to the critic Villeterque, he claims to use this device to make virtue all the more attractive and desirable by portraying it in distress. This defense does not seem to hold up entirely when one examines the theme of vice and virtue that weaves through Sade's fiction.