The psychiatrist Berthold Kihn was a pioneer of Nazi “euthanasia”. As early as 1932 he propagated the killing of mentally handicapped and mentally ill people. In the “Third Reich”, Kihn consequently took a prominent part in the National Socialist genetic health, race and extermination policy. As an expert of the “T-4 Action program”, he was responsible for the mass killing of patients in mental hospitals. The essay outlines Berthold Kihn’s career and asks about the motives and ideological goals that led him to become a medical perpetrator.