Unit 731 remains one of the most controversial and disturbing examples of the misuse of modern science for military purposes during the twentieth century. Established by the Imperial Japanese Army during the Second Sino-Japanese War and World War II, Unit 731 operated under the official title of the Kwantung Army’s Epidemic Prevention and Water Purification Department while secretly conducting biological warfare research and human experimentation in occupied Manchuria. This study reconstructs the history of Unit 731 by examining its origins, organizational structure, scientific activities, ethical violations, and postwar legacy. Through historical analysis of archival materials, survivor testimonies, war crimes records and previous academic studies, this research explores how medical knowledge was transformed into instruments of violence and how political circumstances shaped postwar accountability. Particular attention is given to the role of the United States’ decision to obtain biological warfare data from former Unit 731 personnel rather than prosecute all involved individuals, contributing to decades of controversy regarding justice and historical memory. The study argues that Unit 731 represents not only a military atrocity but also a profound crisis in scientific ethics, demonstrating the dangers of separating technological advancement from human rights, morality and legal responsibility.