Just before Hecate exits the scene, she says "And you all know, security is mortals' chiefest enemy" (III.v.32-33). Here Hecate refers to one of the themes of the play--people's desire to feel safe often leads to their downfall. Macbeth has been afraid that the witches' prophecy would not come true; his insecurity in part drove him to murder King Duncan to secure his place on the throne. After this, Macbeth continues to commit ill acts to maintain his sense of security in his new position. Macbeth begins to see friends as enemies, and he is suspicious of everyone. His sense of insecurity clouds his mind and his better judgment, ultimately leading to his downfall.