Schneider is said to exemplify the descent of a distinguished academic psychiatrist into the Nazi worldview. Some described him as having once shown great empathy in his psychiatric rehabilitation work, and having a great idealism about transforming the 'horror' of psychiatric patients thought to be regressed, isolated and backward. He would sometimes put forward two possible ways of helping a patient – one of them 'work therapy', and the other to sterilize or kill them.
Schneider joined the Nazi Party in 1932. He defined and elaborated the psychological assumptions of Nazi ideology and science. He coined the term ''national therapy'' for ethnic cleansing: ridding the populace of genetic and blood contaminants threatening the psychological and physical health of the German/Aryan population. He collected the brains of murdered Jews, mentally handicapped children, and other victims, for research in his clinic and for instruction. He taught a technique of replacing spinal fluid with air, to get clearer x-rays of the brain.
Schneider, along with Konrad Zucker, helped Heidelberg become one of the two leading training centres for the killing of children for theoretically scientific purposes, which went on at thirty clinics for three years.
