A 1920 book by Alfred Hoche and Karl Binding, Allowing the Destruction of Life Unworthy of Life was the foundational text justifying, authorizing, and generating state murder:

"If one thinks of a battlefield covered with thousands of dead youth and contrasts this with our institutions for the feebleminded with their solicitude for the living patients—then one would be deeply shocked by the glaring disjunction between the sacrifice of the most valuable possession of humanity on the one side, and on the other the greatest care of beings who are not only worthless but even manifest negative value."

The message was clear: If the state is willing to sacrifice the lives of its soldiers, why should so many resources be expended to keeping mental patients alive? If the state is willing to kill its own soldiers (its most valuable possession), why can’t it also kill human beings who are “worthless?”

Beginning with this syllogism, we understand the ideological foundation of the Holocaust: why Hitler and the Nazis believed that Nations have the Right to Kill (Koenigsberg, 2009)
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AMAZON'S DESCRIPTION OF THE BOOK: Many people do not realize that the Germans were methodically killing fellow Germans before they were killing Jews, gypsies, and dissidents. 'Action T4' was a medical program that quietly whisked disabled and mentally ill people for extermination. Germans of all ages were targeted. Hundreds of thousands received 'treatment.'

Fewer people know that the philosophical foundations for the Nazi actions were laid many years earlier, even before the National Socialist party was created. In a sober, academic discussion, professors Karl Binding and Alfred Hoche argued that there were 'lives unworthy of life' and for the good of society, and indeed, out of compassion for the worthless individuals, such people could be ethically killed. Binding and Hoche's book was a turning point in German culture and served as a catalyst for the T4 program, which itself was a precursor to the Holocaust.