My research examines the case of Joseph Vacher, one of the most prolific serial murderers in French history, as a micro- historical study to analyze the evolution of criminal theory and application of the insanity defense over the course of the Belle Époque, as French judicial systems and medico-legal experts attempted to cope with the emerging psychiatric distinction between mental illness and personality disorders. Historically, attempts to explain seemingly unmotivated homicides left a narrow margain for mitigating factors, aside from pleas of insanity. The success of such pleas reflected the conviction that these crimes could only result from severe mental incapacity. Nevertheless, in the late nineteenth century, there emerged a new medical perspective, the sadism diagnosis. Those involved in the realm of criminal behavior began to entertain the possibility that certain individuals might commit violent acts in pursuit of pleasure while maintaining full command of their reason.