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"I will 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." - Martin Luther King, Jr.

ETA: Apparently this is falsely, in part, attributed to him. But I still stand by the words.