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Glamis thou art, and Cawdor; and shalt be What thou art promised: yet do I fear thy nature; It is too full o' the milk of human kindness To catch the nearest way: thou wouldst be great; Art not without ambition, but without The illness should attend it: (1.5.1)
Here's another count against ambition: After reading the letter from her husband (which recounts the witches' prophesy), Lady Macbeth's thoughts immediately turn to murder. Problem: Macbeth has ambition, but he doesn’t have the nerve to see it through. Luckily Lady Macbeth is man enough for both of them.