“I’m wondering whose side I ought to be on. I’m getting very sorry for the Devil and his disciples such as the good Le Chiffre. The Devil has a rotten time and I always like to be on the side of the underdog. We don’t give the poor chap a chance. There’s a Good Book about goodness and how to be good and so forth, but there’s no Evil Book about evil and how to be bad. The Devil has no prophets to write his Ten Commandments and no team of authors to write his biography. His case has gone completely by default. We know nothing about him but a lot of fairy stories from our parents and schoolmasters. He has no book from which we can learn the nature of evil in all its forms, with parables about evil people, proverbs about evil people, folk-lore about evil people. All we have is the living example of the people who are least good, or our own intuition.
‘So,’ continued Bond, warming to his argument, ‘Le Chiffre was serving a wonderful purpose, a really vital purpose, perhaps the best and highest purpose of all. By his evil existence, which foolishly I have helped to destroy, he was creating a norm of badness by which, and by which alone, an opposite norm of goodness could exist. We were privileged, in our short knowledge of him, to see and estimate his wickedness and we emerge from the acquaintanceship better and more virtuous men.”

Source: The Story of My Life, Part III, Ch. 5: Literary Style

Source: Paradise Lost

[Morgan, Forrest, Shakespeare—the Man, published in the Prospective Review, July 1853, The works of Walter Bagehot, vol. 1, 1891, Hartford, Connecticut, Travelers Insurance Company, https://babel.hathitrust.org/cgi/pt?id=njp.32101064786716;view=1up;seq=373, 265–266 of 255–302]
Shakespeare—the Man (1853)

Source: The Critic as Artist (1891), Part I

Said to Winston Churchill in Tehran, November 1943, as quoted in Fallen Eagle: The Last Days of the Third Reich (1995) by Robin Cross, p. 21
Contemporary witnesses

"Making America great means exposing 'W'," http://praag.org/?p=21693 Praag.org, February 20, 2016.
2010s, 2016

Interview with "El País", 2009.

Source: Gone Girl

paraphrased variant:
We should not be simply fighting evil in the name of good, but struggling against the certainties of people who claim always to know where good and evil are to be found.
Source: Hope and Memory: Reflections on the Twentieth Century (2003), Ch. 5 : The Past in the Present, p. 195