Who is responsible for the death of King Duncan in “Macbeth”, by William Shakespeare?

GCSE: English Literature

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The main protagonist, Macbeth himself, is an obvious starting point, as it is he who physically murders King Duncan.
Our initial view of Macbeth should be positive enough, as he is presented as a brave and noble warrior, fighting for king and country, “Like Valour’s minion carved out his passage”, Act 1, Scene 2, 19.
However, we also learn, quite quickly, of Macbeth’s ambition. In Act 1, Scene 3, when Macbeth first hears the witches’ predictions, he might be expected either to dismiss them as superstitious nonsense, or to welcome them with open arms. In fact, he does neither; he starts, or flinches, and looks frightened. Banquo has cause to question Macbeth, “Good sir, why do you start and seem to fear / Things that sound so fair?”, Act 1, Scene 3, 50-51. The implication is that perhaps Macbeth is already considering some dark deed against King Duncan and he perceives that the witches’ have read his mind.
Lady Macbeth, too, hints that Macbeth wishes to be more influential. She says of him, “Thou wouldst be great / Art not without ambition”, Act 1, Scene 5, 16-17.
Macbeth, himself, when considering the murder of King Duncan, speaks of his own “Vaulting ambition”, Act 1, Scene 7, 27.
Macbeth strives to be noble and courageous, and to do the right thing. Lady Macbeth manipulates him into killing Duncan, by preying on his fear of being seen to be a coward, and Macbeth, foolishly, accepts her argument, rather than standing up for what his conscience tells him is the honourable thing to do.
In partial mitigation, perhaps, it is clear that Macbeth is a man of conscience, and he struggles with that conscience right up until Duncan’s murder. He recognises that killing the king is immoral, and he agonises about it beforehand, “First, as I am his kinsman and his subject / Strong both against the deed”, Act 1, Scene 7, 13-14. He even decides that he will not go through with the murder, “We will proceed no further in this business”, Act 1, Scene 7, 31 – but his wife cajoles him into committing the murder anyway. Macbeth is also racked with guilt afterwards, “Methought I heard a voice cry, ‘Sleep no more / Macbeth does murder sleep’”, Act 2, Scene 2, 35-36.
Also in Macbeth’s defence, perhaps, is that he is influenced by the “supernatural”, more so than any of the other characters. This may, of course, simply be because he is more superstitious than any of the other characters, but he nevertheless perceives that he sees a dagger when he is on his way to kill Duncan, “Is this a dagger which I see before me, / The handle toward my hand?”, Act 2, Scene 1, 34-35, and he perceives that he hears a voice cry after he has killed Duncan, as described above.
Lady Macbeth plays a hugely significant role in morally compelling Macbeth to go through with the killing of King Duncan.
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