The eleventh and early twelfth century have traditionally been seen as a period when wars were an endemic feature of life and society. Historians have amply analysed the many material, social, and political aspects of war in this period. The psychological impact of war on warriors has been given less attention, however. This is no doubt due to the abundant methodological problems in applying psychological theories developed in – and often contingent on – the culture and society of twentieth and twenty-first century Western society. Nevertheless, human experiences of war seem to share some universal characteristics and the medieval sources support at least a reflection on the impact of the atrocities of war on the mentality of medieval warriors. One such case is the possible impact of the horrors of the First Crusade on knights such as Thomas de Marle. In our main sources for the life of Thomas, the writings of Abbot Guibert of Nogent and Abbot Suger of Saint-Dénis, Thomas is characterised as the most evil man of his generation and as a warrior wholly given over to gratuitous violence and torture for no other reason than his own deranged pleasure.