“Ernst Roehm was a thug. He was a brutish and zealous believer in National Socialism. If anything, he was more radical than Hitler. He believed in the elimination of anyone connected with the old order, businessmen, office holders of any kind. He became the leader of the SA, the Storm Troopers, whose terror tactics had helped make Hitler the leader of Germany.”

Quoted in A. J. Sylvester's diary entry (4 September 1936), Colin Cross (ed.), Life with Lloyd George. The Diary of A. J. Sylvester 1931-45 (London: Macmillan, 1975), p. 148
Later life

‘Once again, I feel I have something to say’ Interview, Page 2 http://www.indianexpress.com/news/once-again-i-feel-i-have-something-to-say/471304/2 Indian Express, Jun 07, 2009.

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: Becoming Hitler: The Making of a Nazi (2017), pp. 49-50

Dissertation for doctor of philosophy in christian education (May 25, 1991)

To Leon Goldensohn, April 6, 1946, from "The Nuremberg Interviews" by Leon Goldensohn, Robert Gellately - History - 2004

Source: Der Fuehrer, Hitler’s Rise to Power (1944), p. 501

Source: Managing the Non-Profit Organization: Principles and Practices

Summation for the Prosecution, July 26, 1946
Quotes from the Nuremberg Trials (1945-1946)