In a Shakespearean tragedy the protagonist rises to great heights, but through the choices and actions taken, the character is condemned to a catastrophic fall. This is perfectly illustrated in Macbeth. As Macbeth ascends he gains the Thane of Glamis, the Thane of Cawdor, and the throne of Scotland. From this plateau there is no higher earthly point for Macbeth to fall from.
Macbeth's first crucial occurrence as king was the escape of Fleance (Banquo's son) from the murderers. The second murderer even knew this was a loss to Macbeth for he says, "We [the murderers] have lost best half of our affair" (III, iv, 21). This event reinforces the witches' prophesy proclaiming, "Thou [Banquo] shall get kings, though thou be none..."(I, iii, 65). Translated, Macbeth will inevitably die, lose the line of kingship, and lose the chance to be head of dynasty. The end of this conflict begins the start of Macbeth's fall from his high estate.
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