The main message of Macbeth, a tragedy written by William Shakespeare, is "power corrupts". The play explores the consequences of unchecked ambition and the corrupting influence of power. Macbeth, the main character, becomes increasingly ruthless and paranoid as he gains power through immoral means. Ultimately, his desire for power leads to his downfall and tragic end. The play suggests that power should be used responsibly and with caution, and that unchecked ambition can have destructive consequences. Therefore, the correct answer is C. Power corrupts.