Not by coincidence did the transition from compulsory sterilization to murder occur around the time of war outbreak in 1939. In the Nazis' view, the elimination of "dead weights" and "useless eaters" was supposed to counteract the "negative selection" resulting from war (death or mutilation of the healthy, survival of the sick). On a practical level, the underlying motive for this mass murder was to economize on hospital beds, medical staff, food, medicine, etc. and, thus, free these resources for the war economy.
The Nazis' extermination of those "unworthy to live" - incorrectly called "euthanasia" (Greek: good death) or "mercy killing" - began with sick children. From autumn 1939, "malformed and idiotic children" were killed in special "children's wards." Shortly afterward, the killing of adult asylum patients started, prompted by an "authorization" by Hitler antedated to 1 September 1939. The operation was called "T4" after the seat of the euthanasia front organization at Tiergartenstraße 4 in Berlin. A considerable part of the patients in psychiatric institutions of the German Reich were selected for "euthanasia" by paid "experts," who used questionnaires, and were gassed in killing institutions such as Hartheim Castle near Linz. Cause and place of death were systematically falsified in the death certificates and in information given to relatives and authorities. "Operation T4" also involved patients of smaller, mostly clerical, institutions and went beyond the mentally sick to include inmates of nursing homes and senior citizens' homes.
From the Viennese Heil- und Pflegeanstalt Steinhof alone, approximately 3,200 patients - among them 400 Jews - were transported to Hartheim via intermediary institutions like Niedernhart and Ybbs an der Donau in the years 1940/41. In the course of "Operation T4," over 18,200 mentally sick or handicapped individuals were murdered in the Hartheim killing institution. After the official termination of the operation in August 1941, Hartheim continued to serve as a gassing site for at least 8,000 prisoners from the concentration camps at Dachau, Mauthausen, and Gusen ("Operation 14f13") and for mentally ill Ostarbeiter (laborers from Eastern Europe) who could no longer perform any work.
Even though the transfer of patients to euthanasia clinics was discontinued in August 1941, not least due to protests by the Church, euthanasia was independently continued in various ways in the individual institutions.