It's difficult to determine from moment to moment in Shakespeare's Macbeth whether Macbeth is motivated by fear, ambition, or love, whether any of these motivations is rational or irrational, and which of the decisions that Macbeth makes based on these motivations is rational or irrational.
The most significant decision that Macbeth makes in the play is to murder Duncan. Everything else that happens in the play occurs as a result of that choice.
Why does Macbeth kill Duncan, and what is irrational about his choice to do so?
If Macbeth believes the witches' prophecies—which he apparently does—why wouldn't Macbeth consider murdering Duncan as simply fulfilling that prophecy and a perfectly rational thing to do? Why wouldn't Macbeth consider Duncan's visit to his castle as a fortuitous circumstance to help him fulfill the prophecy, and that taking advantage of that circumstance is a perfectly rational thing to do?
MACBETH: If chance will have me king, why, chance
may crown me
Without my stir....
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