In November 1940 Stangl was promoted to run security at Schloss Hartheim, one of the hospitals used in the Nazi’s T4 euthanasia program. This program aimed to eliminate mentally and physically disabled people, who were deemed an economic burden on society. T4 referred to the headquarters address—Tiergartenstrasse 4—of the two most secret Nazi programs: the murder of disabled people under the guise of “mercy killings” and later, the “Final Solution.” Some of the people associated with the euthanasia program had good intentions, seeing death as a merciful release both for people with severe disabilities and their families. However, in practice, it became a program of organized murder in which “medical” functionaries at T4 decided on the life or death of tens of thousands of people.

Stangl claimed that at first, he was horrified by the program and refused the assignment. However, he changed his mind because the assignment would free him both from his hated boss, Prohaska, and his pending disciplinary action. Additionally, Stangl was told that such euthanasia was already legal in the US and USSR and that medical staff would administer careful examinations of the “patients.”

At Hartheim, Stangl first met Captain Christian Wirth (called “the savage Christian” for his brutality), who was the roving director of the dozen T4 facilities in Germany.