Shakespeare’s use of the dagger in these lines from Macbeth is rich with symbolism and reflects the protagonist's inner turmoil. The following two interpretations can be used to explain its use:
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Macbeth's guilt for thinking about killing the king has triggered its appearance - The dagger symbolizes Macbeth's guilt and ambition as he contemplates the murder of King Duncan. Its appearance, despite being a hallucination, indicates that his conscience is deeply troubled by the immoral thoughts and desires he harbors.
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The dagger is a message to Macbeth that he should act on his impulses - The dagger, an instrument of murder, serves as a powerful symbol for Macbeth’s growing determination to follow through with his dark intentions. It reflects his inner conflict between ambition and morality while also pushing him towards the fateful decision to commit murder.
These interpretations highlight the psychological complexities of Macbeth's character and the themes of guilt and ambition that are central to the play.