Why did Macbeth kill Duncan?
Macbeth is put on the path to murder when he encounters the witches, who greet him with three titles: "thane of Glamis," "thane of Cawdor" and "king hereafter." At this point in the play, he has not yet received news that he's been granted the thaneship of Cawdor. This would come soon after, when Ross and Angus arrive after the witches depart.
Prophecy is a critical theme in Macbeth. It is prophecy that sets in motion Macbeth's usurpation of Duncan, and Macbeth's belief in prophecy also drives the death of Banquo. Still later in the play, Macbeth is given more prophesies, this time regarding his demise.
Ultimately, though, it is Macbeth's own ambition and his desire for kingship (a desire which the Lady Macbeth shares) which is responsible for the murder of Duncan. For example, in Act 1 Scene 4, Macbeth refers...
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