For context this is the full quote, where MLK Jr. condemns riots but also equally condemns the conditions that cause people to riot: inequality, injustice, lack of humanity, lack of progress.

Let me say, as I’ve always said, and I will always continue to say, that riots are socially destructive and self-defeating. I’m still convinced that nonviolence is the most potent weapons available to oppress[ed] people in their struggle for freedom and justice. I feel that violence will only create more social problems than they will solve, that in a real sense, it is impractical for the Negro to even think of mounting a violent revolution in the United States. So I will continue to condemn riots and continue to say to my brothers and sisters that this is not the way. Continue to affirm that there is another way.

But at the same time, it is as necessary for me to be as vigorous in condemning the conditions which cause persons to feel that they must engage in riotous activities, as it is for me to condemn riots. I think America must see that riots do not develop out of thin air. Certain conditions continue to exist in our society, which must be condemned as vigorously as we condemn riots. And in the final analysis, a riot is the language of the unheard. And what is it that America has failed to hear? It has failed to hear that the plight of the Negro poor has worsened over the last few years. It has failed to hear that the promises of freedom and justice have not been met. And it has failed to hear that large segments of white society are more concerned about tranquility and the status quo than about justice, equality, and humanity. So in a real sense, our nation’s summer’s riots are caused by our nation’s winters of delay. And as long as America postpones justice, we stand in the position of having these recurrences of violence and riots over and over again. Social justice and progress are the absolute guarantors of riot prevention.