King Duncan's death in Shakespeare's "Macbeth" is attributed to the manipulative influence of Lady Macbeth, the ambition of Macbeth, and Duncan's own misplaced trust. Lady Macbeth's desire for power drives her to persuade Macbeth to commit the murder, showcasing her pivotal role in the plot. Macbeth, although initially hesitant, succumbs to her pressure and ultimately carries out the act. Additionally, Duncan's trusting nature makes him vulnerable, as he fails to recognize the deceit around him, particularly from Macbeth and Lady Macbeth. The interplay of these factors illustrates the tragic consequences of ambition and betrayal in the play.