How crucial is the second meeting of Macbeth with the witches ?
Macbeth’s first meeting with the witches sets him up for his “triumph” of becoming the king of Scotland. It gives him the impetus to take action, murdering Duncan, as well as Banquo. The prophecy was specific to the outcome, but says nothing about the method. Macbeth decides he must himself take charge of the method, not trusting to fate to lead him to his destiny unaided.
The second meeting, however, is misleading. Each prophecy is interpreted by Macbeth as a guarantee that he cannot fail, when in fact it foretells his destruction. Macbeth cannot be killed by anyone born of woman. Macbeth does not understand the witches’ definition of “born” as a natural birth. Macduff was born, but it was not a natural birth but a caesarean section. He will never be vanquished until Great Birnam Wood marches to Dunsinane Hill. Macbeth takes this literally and completely ignores the word “until.” When Macduff’s forces use tree branches to hide their movements, it seems that the woods are marching. The second prophecy was meant as a warning that Macbeth completely missed. Instead, it made him overconfident and led to his death.