“I almost think it is the ultimate destiny of science to exterminate the human race.”

A characteristic saying of Russell, reported by Aldous Huxley in a letter to Lady Ottoline Morrell dated 8 October 1917, as quoted in Bibliography of Bertrand Russell (Routledge, 2013)
1910s

Je crois qu'il faut presque toujours un coup de folie pour bâtir un destin.
Les yeux ouverts: entretiens avec Matthieu Galey [With Open Eyes: Conversations With Matthieu Galey] (1980)

Preface; Previous Attempts Miss the Point.
1930s, On the Rocks (1933)
Context: The extermination of what the exterminators call inferior races is as old as history. "Stone dead hath no fellow" said Cromwell when he tried to exterminate the Irish. "The only good nigger is a dead nigger" say the Americans of the Ku-Klux temperament. "Hates any man the thing he would not kill?" said Shylock naively. But we white men, as we absurdly call ourselves in spite of the testimony of our looking glasses, regard all differently colored folk as inferior species. Ladies and gentlemen class rebellious laborers with vermin. The Dominicans, the watchdogs of God, regarded the Albigenses as the enemies of God, just as Torquemada regarded the Jews as the murderers of God. All that is an old story: what we are confronted with now is a growing perception that if we desire a certain type of civilization and culture we must exterminate the sort of people who do not fit into it. There is a difference between the shooting at sight of aboriginal natives in the back blocks of Australia and the massacres of aristocrats in the terror which followed the foreign attacks on the French Revolution. The Australian gunman pots the aboriginal natives to satisfy his personal antipathy to a black man with uncut hair. But nobody in the French Republic had this feeling about Lavoisier, nor can any German Nazi have felt that way about Einstein. Yet Lavoisier was guillotined; and Einstein has had to fly for his life from Germany. It was silly to say that the Republic had no use for chemists; and no Nazi has stultified his party to the extent of saying that the new National Socialist Fascist State in Germany has no use for mathematician-physicists. The proposition is that aristocrats (Lavoisier's class) and Jews (Einstein's race) are unfit to enjoy the privilege of living in a modern society founded on definite principles of social welfare as distinguished from the old promiscuous aggregations crudely policed by chiefs who had no notion of social criticism and no time to invent it.

The Human: Science versus Humanism (p. 3)
Straw Dogs: Thoughts on Humans and Other Animals (2002)

The Twinkling of an Eye: My Life as an Englishman (1998) Unsourced variant: "Why had I become a writer in the first place? Because I wasn't fit for society; I didn't fit into the system."

Source: The Time Axis (1949), Ch. 3 : The Vision Of Time
Context: I closed my eyes again, thinking of the Face. I had to force my mind to turn around in its tracks and look, for it didn't want to confront that infinite complexity again. The Face was painful to see. It was too intricate, too involved with emotions complex beyond our grasp. It was painful for the mind to think of it, straining to understand the inscrutable things that experience had etched upon those mountain-high features.
"Is it a portrait?" I asked suddenly. "Or a composite? What is the Face?"
"A city," De Kalb said. "A nation. The ultimate in human destiny — and a call for help. And much more that we'll never understand."

Conjectures of a Guilty Bystander (1966)
Context: It is a glorious destiny to be a member of the human race, though it is a race dedicated to many absurdities and one which makes many terrible mistakes: yet, with all that, God Himself gloried in becoming a member of the human race. A member of the human race! To think that such a commonplace realization should suddenly seem like news that one holds the winning ticket in a cosmic sweepstake.

The Almost Perfect State (1921)