“Among others, Doctor Caritens died during a momentary absence of my father, who recommended while stepping into his travelling chariot, to bleed the Doctor a second time. I did as he bid me, although convinced that emetics and opening medicine would cure the patient without fail; the Doctor died, and you may easily imagine the state of my feelings. I had just begun to publish a work on practical physic, but had no heart to finish it after this sad catastrophe. I betook myself again to philosophy. I wished daily to return to Gottingen, if I could do so with honour. I passed three years under such painful circumstances, when my friend Leisewitz invited me to go with him to Berlin, for which purpose his brother-in-law in Brunswick would advance me money to defray my expenses. Without much consideration, I accepted the invitation, and my portmanteau was soon ready.”

Source: The Story of an Hour

Source: The Hidden Oracle

Speech at MIT (1985), referring to his brother Bernard Vonnegut, and the choices available to scientists and the intelligent, to serve humanity, or to betray it, as published in Fates Worse Than Death (1991), Ch. 12
Various interviews
Context: My brother got his doctorate in 1938, I think. If he had gone to work in Germany after that, he would have been helping to make Hitler's dreams come true. If he had gone to work in Italy, he would have been helping to make Mussolini's dreams come true. If he had gone to work in Japan, he would have been helping to make Tojo's dreams come true. If he had gone to work in the Soviet Union, he would have been helping to make Stalin's dreams come true. He went to work for a bottle manufacturer in Butler, Pennsylvania, instead. It can make quite a difference not just to you but to humanity: the sort of boss you choose, whose dreams you help come true.
Hitler dreamed of killing Jews, Gypsies, Slavs, homosexuals, Communists, Jehovah's Witnesses, mental defectives, believers in democracy, and so on, in industrial quantities. It would have remained only a dream if it hadn't been for chemists as well educated as my brother, who supplied Hitler's executioners with the cyanide gas known as Zyklon B. It would have remained only a dream if architects and engineers as capable as my father and grandfather hadn't designed extermination camps — the fences, the towers, the barracks, the railroad sidings, and the gas chambers and crematoria — for maximum ease of operation and efficiency.

Source: Sylvia cartoon strip, p. 106

Act V., Scene II. — (Cornelio).
Translation reported in Harbottle's Dictionary of quotations French and Italian (1904), p. 274.
I Lucidi (published 1549)

As quoted in Riccardo Orizio, Talk of the Devil: Encounters with Seven Dictators, (Walker and Company, 2003), p. 145

Fiction, The Crawling Chaos (1921)

Attempting to quell rumors of Soviet leader Chernenko's ill health, as quoted in "Visiting Soviet Doctor Changes His Statement" in The New York Times (10 February 1985) http://select.nytimes.com/gst/abstract.html?res=F70F14F63C5D0C738DDDAB0894DD484D81.

Quoted by Mark Bonham Carter in his Introduction to the 1962 edition of The Autobiography of Margot Asquith (London: Eyre & Spottiswoode, 1962) p. xxxv.