Macbeth's great mistake, what we might call his "original sin," was to act on his dark impulse to murder Duncan. Fantasizing about achieving one's deepest desire is one thing—crossing a line into killing to achieve it is another.
Macbeth is already coming to his senses, stepping back, and weighing the consequences of his act by the time he has returned to his castle, not long after quelling the rebels in battle. He realizes that Duncan is a good and merciful king who has treated him generously. He wishes to bask for awhile in Duncan's favor and enjoy his status as the hero who killed the traitor. More profoundly, he sees with clear eyes that once he heads down the path of bloodshed, there is no turning back. He reasons that it would be not so terrible to kill Duncan if it simply ended there, but he realizes, too, that if he goes down that road, it is the beginning of what will become a widening circle of death.
Macbeth has regained his senses at this point but has already made another...
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