ObjectiveThis article is the result of twenty years of research on the relationships between psychiatry, crime and justice in the 19th century. This paper focuses in particular on the case of Antoine Léger, found guilty of murder, rape and cannibalism in 1824. The analysis of this particularly horrific case, also characterised by its many repercussions extending into the 20th century, enables the elaboration of the crime and subsequently of the medical case to be followed, as well as the increasing scrutiny of the triggers underpinning human actions. MethodThe analysis of this case via a micro-historic approach is based on a large corpus of judicial archives, medical writings that made use of the case, and press coverage. ResultsThe series of writings that made use of the Léger case show the extent to which the aetiology of perversity and perversion fuelled reflexion through the history of psychiatry. The Léger affair was the subject of numerous diagnoses, initially focusing on Léger's penal responsibility. Etienne Georget and Etienne Esquirol, contemporaries of the affair, thus supported the idea of an act of madness (monomania) despite the fact that Léger was convicted. From the mid-19th century the questioning changed, and it was the nature of the act that focused the attention of physicians, seen as demonstrating a psychopathology of cruelty, and leading on to the hypothesis of sexual perversion proposed by Krafft-Ebing. According to this elaboration, the figure of the werewolf, latent in 1824 and formalised in the second half of the 19th century, and its scholarly form of lycanthropy, appear to serve as conceptual intermediaries able to provide lines of approach to interpret the crime. DiscussionThis exploration has enabled a reappraisal of this case, already examined earlier by the team working with Michel Foucault in the 1970s on a wide-reaching history of the judicial system and psychiatry. It also enables consideration of new research methods for case studies as defined by Jacques Revel and Jean-Claude Passeron in their Penser par cas (Paris, 2005), where cases are seen as liable to redefine normality and its exceptions. The study also brought to light the contributions of anthropology, and in particular the work by Daniel Fabre, with an attempt to distance dominant discourse and apprehend acts and gestures in their own meaning. ConclusionLéger the werewolf has thus made a silent contribution to reflection on perverse acts, and this links two stages in the interpretation of cruel acts: cruelty as a form of immorality, and cruelty as sexual perversion.