What evidence is there related to the idea of fate vs. free will in William Shakespeare's Macbeth?
William Shakespeare's Macbeth was set in medieval Scotland, and all of the characters in the play would have been members of the Roman Catholic Church. Shakespeare himself was writing after the foundation of the Church of England, but on key issues of fate and free will, the theology is similar if not identical. The key issue in Macbeth of free will and fate is to a large degree theological.
Essentially, in Christian theology, all humans participate in Original Sin, but all have the opportunity to benefit from the sacrifice of Christ who died to redeem us from Original Sin, reconciling God to humanity. Our free will is irrelevant to this. The second type of sin is the actual sins we commit after baptism. This is where free will becomes important. We can only justly be blamed for our sins if we are free to choose to commit them or not; if we have no choice, it seems that it would be unjust for God to punish us for them (theologians refer to the problem of divine justice as theodicy). Thus moral choice requires free will. The great struggle, though, is how to reconcile foreknowledge with free will.
In the case of Macbeth, the evidence of such problematic foreknowledge occurs in the speeches of the three sisters who correctly foretell everything that happens in the play. Given that foreknowledge, and Macbeth's awareness of it, to what degree is it possible to blame Macbeth for his actions?
Shakespeare's plot structure emphasizes that at every step of the play Macbeth (aided and abetted by his wife) freely makes the decision to act immorally. The witches do not predict that Macbeth needs to kill Duncan to become king; had Macbeth simply continued to be a good and loyal subject to Duncan, he could well have been appointed to the position. Essentially, the plot structure of the play, and the soliloquies and discussions with Lady Macbeth which illustrate Macbeth's decision-making process, show that even when a long term outcome is foreordained, we still can make choices, using our free will, and thus maintain the compatibility among foreknowledge, free will, and divine justice.