Ultimately, the prophesies of the Witches light the spark of ambition that propels Macbeth to eventually murder Duncan. At the beginning of the play, Macbeth is content. When the witches tell him that he will be Thane of Cawdor and then king, he doesn't believe them:
and to be king
Stands not within the prospect of belief,
No more than to be Cawdor. (I.iii.77-79)
Oh, how things turn quickly once Macbeth realizes that he has suddenly inherited the title of Thane of Cawdor:
Glamis, and thane of Cawdor!
The greatest is behind. (I.iii.125-126)
Already Macbeth has his eye on the crown, and it won't take long before he is willing to stop at nothing to fulfill what he believes is his destiny, thanks to the Witches. He decides to share the news with his wife so that she "might’st not lose the dues of / rejoicing, by being ignorant of what greatness is / promised thee" (I.v.11-13). Once Lady Macbeth hears of the Witches' predictions, she sets out to strengthen the likelihood of Macbeth's kingship herself,...
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