“Those who abjure violence can only do so by others committing violence on their behalf.”

Direct Action (1912)
Context: Those who, by the essence of their belief, are committed to Direct Action only are — just who? Why, the non-resistants; precisely those who do not believe in violence at all! Now do not make the mistake of inferring that I say direct action means non-resistance; not by any means. Direct action may be the extreme of violence, or it may be as peaceful as the waters of the Brook of Siloa that go softly. What I say is, that the real non-resistants can believe in direct action only, never in political action. For the basis of all political action is coercion; even when the State does good things, it finally rests on a club, a gun, or a prison, for its power to carry them through.

Citizenship Papers (2003), The Failure of War

Irshad Manji: Islam Needs Reformists, Not 'Moderates' http://www.wsj.com/articles/SB10001424052748703992704576305412360432744 The Wall Street Journal (7 May 2011)

This has commonly been attributed to Orwell but has not been found in any of his writings. Quote Investigator http://quoteinvestigator.com/2011/11/07/rough-men/ found the earliest known appearance in a 1993 Washington Times essay by Richard Grenier: "As George Orwell pointed out, people sleep peacefully in their beds at night only because rough men stand ready to do violence on their behalf." The absence of quotation marks indicates Grenier was using his own words to convey Orwell's opinion; thus it may have originated as a paraphrase of his statement in "Notes on Nationalism" https://www.mtholyoke.edu/acad/intrel/orwelnat.htm (May 1945): "Those who "abjure" violence can only do so because others are committing violence on their behalf." There are also similar sentiments expressed in an essay which Orwell wrote on Rudyard Kipling, quoting from one of Kipling's poems: "Yes, making mock o' uniforms that guard you while you sleep." In the same essay Orwell also wrote of Kipling: "He sees clearly that men can only be highly civilized while other men, inevitably less civilized, are there to guard and feed them."
Misattributed

2010s, 2015, Speech on extremism (20 July 2015)

29 June 2005
Opposition to the proposed Reconciliation and Unity Commission

This passage contains some phrases King later used in "Where Do We Go From Here?" (1967) which has a section below.
1950s, Loving Your Enemies (Christmas 1957)
Variant: 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.
Source: Mentioned in "Out of Osama's Death, a Fake Quotation Is Born" by Megan McArdle, The Atlantic (May 2011) http://www.theatlantic.com/national/archive/2011/05/out-of-osamas-death-a-fake-quotation-is-born/238220/, and widely distributed on twitter http://twitter.com/#!/jmadly/status/65314784136011776 as a quote of King, after the death of Osama bin Laden, the first sentence is one written by Jessica Dovey http://i.imgur.com/cqtjw.jpg on her Facebook page, which became improperly combined by others with genuine statements of King, whom she quoted, and which occur in Strength to Love (1963), Ch. 5 : Loving your enemies, and in Where Do We Go from Here : Chaos or Community? (1967), p. 62.
For the full story see "Anatomy of a Fake Quotation" by Megan McArdle, The Atlantic (May 3, 2011) http://www.theatlantic.com/national/archive/2011/05/anatomy-of-a-fake-quotation/238257/ and for the Facebook version of the quote see Did Martin Luther King, Jr. say that “I will not rejoice in the death of one, not even an enemy”? at skeptics.stackexchange.com http://skeptics.stackexchange.com.
Context: 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. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction.
Context: Let us move now from the practical how to the theoretical why: Why should we love our enemies? The first reason is fairly obvious. 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. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction. So when Jesus says "love your enemies," he is setting forth a profound and ultimately inescapable admonition. Have we not come to such an impasse in the modern world that we must love our enemies-or else? The chain reaction of evil-Hate begetting hate, wars producing more wars-must be broken, or we shall be plunged into the dark abyss of annihilation.
Context: I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. 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.