Karl Willig was born in the town of Gräveneck near Weissburg in the state of Hesse in 1894. Since July 1941, he had worked as an orderly or deputy head nurse in the state hospital and reformatory of Hadamar.
Since the beginning of the decentralized »euthanasia« programme in Hadamar from the middle of 1942, he, together with other nurses and chief physician Adolf Wahlmann, had been involved in the selection and killing of institutional patients and Jewish »half-breeds« by overdoses of medicine or food deprivation in the institution. In 1944/45, Willig gave Polish and Soviet forced labourers suffering from tuberculosis, who had been taken to Hadamar to be killed, lethal injections or tablets. At Hadamar, Willig was also responsible for the subsequent incineration of the victims.
According to post-war statements, Willig was regarded as a particularly brutal caregiver, who also abused patients. Together with other caregivers, nurses and doctors, Willig was indicted by the US military court in Wiesbaden in October 1945 for the killings of about 500 forced labourers between the summer of 1944 and March 1945 (»Hadamar Trial«). Willig was sentenced to death and executed by hanging in Bruchsal prison on 14 March 1946.