Why does Macbeth kill King Duncan?
Macbeth's desire to usurp the throne increases steadily after the three witches tell him that he will be king. Lady Macbeth forcefully encourages Macbeth to murder Duncan, and eventually he gives over to her appeal and his own desire for power. The question posed to Macbeth by the witches' prophecy is whether to let the prophecy come true on its own or to take fate into his own hands, which he does.
Macbeth first thinks about killing King Duncan after the the three witches prophesize that he will one day be king himself. After he hears this, he asks himself why he contemplates "that suggestion / Whose horrid image doth unfix my hair / And make my seated heart knock at my ribs, / Against the use of nature?" The "suggestion" he is tempted by is the idea that he should kill Duncan, which, as he says, is a sinful thought to have, and unnatural, or "Against the use of nature." He subsequently says in act 1, scene 3, that, "If chance will have me king, why chance may crown me, Without my stir." In other words, if it is his fate to be king, then he should simply let fate take its course and does not need to "stir," or help it along. This implies firstly that he was previously thinking about intervening and helping fate along by killing the king, and secondly that he has now decided that he will not kill the king, but let fate unwind as it will.
Macbeth does not find it easy to simply sit back...
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Macbeth kills Duncan becaide he tells his wife of the witches Prophecy and is convinced by lady macbeth that he must kill him in order to ptobe his manhood.