One could argue that the major theme of act 2 is the danger of ambition. Ambition, like money, has the remarkable power to make people do things they don't really want or need to do. And that's what happens here in relation to Macbeth.
Showered with honors and secure in the love of his king, Macbeth doesn't need to kill Duncan, the man he's previously served with such loyalty and distinction. And yet in act 2, that's precisely what he does. Why? Because his soul has become corrupted by ambition. In presenting Macbeth with their prophecies, the Weird Sisters planted the demon seed of ambition in his mind, and it has now blossomed into a poisonous weed that, from now on, will determine his every course of action.
That's not to say that ambition is given to us in act 2 as necessarily a bad thing. It's rather the nature of that ambition and how it can drive us to commit unspeakable acts. Macbeth is already an ambitious man to begin with, but the difference is that such ambition was always...
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