How is Macbeth's conflict intensified by the events of scene 4?

During the events of Scene IV, Duncan announces that his successor will be his own son, Malcolm. This obviously angers Macbeth because he thought the next king would be him, as prophesied by the three witches. This is the moment when he decides for certain that he has to kill Duncan if he wants to take the throne for himself. He concocted a plan along with his wife, and together they murdered the king.