I focused on Shakespeare's use of fate as a concept in his plays for my final assignment, so I couldn't help but notice the role that fate plays in Macbeth, particularly in the last scenes. Generally we think of fate as something that's predetermined and fixed- but the way Shakespeare muses on it, it seems that fate is susceptible to outside forces and a singular fate rests in the hands of the individual.
Macbeth believes that he is not in control of his own fate, and this is apparent through his blind following of the prophesies given by the witches. He is only skeptical of their predictions for moments during their first encounter, until they are proven right when he is pronounced Thane of Cawdor. After that, Macbeth believes that everything the witches say will happen... yet he is unable to recognize his role in these prophecies. At first, he makes it happen. He kills Duncan and defends his title as King; instead of waiting to ascend the throne (because the witches told him it would happen!) he speeds up the process with help from Lady Macbeth.
It seems at first that the witches are predicting Macbeth's entire future when in reality, they are reliant on his actions for their predictions to become reality. The witches aren't the "evil doers" in Macbeth past taking on the role of the instigator, perhaps. However, they didn't promote murder, lying or cheating, they merely told Macbeth of his future. If Macbeth sat idly by and did nothing to make himself king, perhaps it wouldn't have happened.
The roles that the witches and Macbeth play in his own fate, destiny and (eventual) demise are both interwoven and confusing. Shakespeare has created witches- scary, magical beings as a representation of the power we all have to manipulate and even mutate our own futures. Without the witches, Macbeth would have never assumed he had the power to become king- it wouldn't have crossed his mind as an attainable future. However, it is at the hands of Macbeth (and indeed, Lady Macbeth) and not a magical, supernatural force that allows him to ascend to the Scottish throne!
The last lines the witches have (in unison, rather than as independent beings, mind you) is in Act 4 Scene 1, "Show his eyes and grieve his heart, Come like shadows, so depart" (4.1, 126-127)- they summon the last apparition from their cauldron; the band of eight kings and Banquo. This certainly fuels Macbeth's paranoia and emotions, but there is an entire Act left of the play where Macbeth's fate unfolds without another utterance from a supernatural force! Clearly Shakespeare is asking us as his audience to question the role of fate and destiny in Macbeth; the powers all of us have individually to create our futures (the prices way pay for the destinies we choose) and the ability all of us have to be the "witch" to plant the "seed of evil".