I don't doubt the memory just because there's nothing more dangerous than a teacher who can't recognize a Bible quote and who then attributes modern meaning to the speaker. MLK was a reverend so he obviously quoted the Bible a lot and the Bible can have some violent imagery.

But I can't see the man who said this advocating violence of any kind...

"Nonviolence is the answer to the crucial political and moral questions of our time: the need for man to overcome oppression and violence without resorting to oppression and violence. Man must evolve for all human conflict a method which rejects revenge, aggression and retaliation. The foundation of such a method is love."