Based on his Nobel acceptance speech, Martin Luther King Jr. seems to believe that human beings are something in between basically good and bad.
In his speech, King acknowledges the brutality of some humans towards others, particularly in the context of racism and war. He states, "I refuse to accept the idea that man is mere flotsam and jetsam in the river of life, unable to influence the unfolding events which surround him. I refuse to accept the view that mankind is so tragically bound to the starless midnight of racism and war that the bright daybreak of peace and brotherhood can never become a reality."
However, he also expresses a belief in the potential for human goodness and progress, particularly through the activism and dedication of individuals fighting for justice and equality. He goes on to say, "I have the audacity to believe that peoples everywhere can have three meals a day for their bodies, education and culture for their minds, and dignity, equality, and freedom for their spirits. I believe that what self-centered men have torn down, other-centered men can build up."
Therefore, Martin Luther King Jr. seems to believe that human beings are capable of both great evil and great good, and that it is up to individuals to choose to work towards progress and justice in the world.