“I hate a fellow whom pride or cowardice or laziness drives into a corner, and who does nothing when he is there but sit and growl. Let him come out as I do, and bark.”

Principle attributed to Popper by Ryszard Kapiscinski in New York Times obituary, 1995.
Misattributed
Source: https://www.nytimes.com/1995/01/01/magazine/lives-well-lived-karl-popper-the-philosopher-as-giantslayer.html

Song 16: "Against Quarrelling and Fighting".
1710s, Divine Songs Attempted in the Easy Language of Children (1715)

Quoted by Jan Lundius, in Does WFP Deserve the Nobel Peace Prize?, Inter Press Service News Agency, (December 2020)

'Where Do We Go From Here?" as published in Where Do We Go from Here : Chaos or Community? (1967), p. 62; many statements in this book, or slight variants of them, were also part of his address Where Do We Go From Here?" which has a section below. A common variant appearing at least as early as 1968 has "Returning violence for violence multiplies violence..." An early version of the speech as published in A Martin Luther King Treasury (1964), p. 173, has : "Returning hate for hate multiplies hate..."
1960s
Source: A Testament of Hope: The Essential Writings and Speeches
Context: The ultimate weakness of violence is that it is a descending spiral, begetting the very thing it seeks to destroy. Instead of diminishing evil, it multiplies it. Through violence you may murder the liar, but you cannot murder the lie, nor establish the truth. Through violence you may murder the hater, but you do not murder hate. In fact, violence merely increases hate. So it goes. … Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.

Under Fire (1916), Ch. 24 - The Dawn
Context: I tell them that fraternity is a dream, an obscure and uncertain sentiment; that while it is unnatural for a man to hate one whom he does not know, it is equally unnatural to love him. You can build nothing on fraternity. Nor on liberty, either; it is too relative a thing in a society where all the elements subdivide each other by force.
But equality is always the same. Liberty and fraternity are words while equality is a fact. Equality should be the great human formula — social equality, for while individuals have varying values, each must have an equal share in the social life; and that is only just, because the life of one human being is equal to the life of another. That formula is of prodigious importance. The principle of the equal rights of every living being and the sacred will of the majority is infallible and must be invincible; all progress will be brought about by it, all, with a force truly divine. It will bring first the smooth bed-rock of all progress — the settling of quarrels by that justice which is exactly the same thing as the general advantage.

Paris Review interview (1958)

Essay in the anthology The War Poets (1945) edited by Oscar Williams

"I am not I", from Lorca and Jiménez: Selected Poems, chosen and translated by Robert Bly (Boston: Beacon Press, 1973), p. 77