The study examines the massacre of the garrison of the castle of Kalmar in 1525 and its causes. The goal of the investigation has been on the one hand to elucidate the most probable sequence of events, and on the other to identify as many different plausible explanations to the massacre as possible. For this end, four contemporary sources have been examined and compared in their details. To enable a broad basis for interpretation, the author has consulted various works on the themes of warfare, soldiers and violence in the late middle ages and the sixteenth century. Apart from political aspects, the study highlights social and cultural conditions as important factors in explaining how the massacre could be carried out without provoking any single part of society.