The notion of sexual sadism emerged from nineteenth-century alienist attempts to imagine the pleasure of the torturer or mass killer. This was a time in which sexuality was mapped to social progress, so that perversions were always related either to degeneration or decadence. These ideas were internalized in later Freudian views of the drives within the self, and of their repression under the demands of modern European civilization. Sadism was always presented as the barbarous past that lurked within each of us, ready to burst forth into murderous violence, crime, anti-Semitism, and finally genocide. This idea maintained its currency in European thought after the Second World War as Freudian-influenced accounts of the history of philosophy configured the Marquis de Sade as a kind of Kantian superego in a framework that viewed the Western Enlightenment as unraveled by its own inner demons. In this way, a straight line was imagined from the late eighteenth century to the Holocaust. These ideas have had an ongoing legacy in debates about sexual perversion, feminism, genocide representation, and historical memory of Nazism. However, recent genocide research has massively debunked assumptions that perpetrators of mass violence are especially sexually motivated in their cruelty. This book considers how the late twentieth-century imagination eroticized Nazism for its own ends, but also how it has been informed by nineteenth-century formulations of the idea of mass violence as a sexual problem.