I have to write an essay on this. Can you please give me some ideas, like what the issues are, and how I can write how the audience may interpret the issues.
In my experience, the greatest division in thought about Macbeth is something mentioned by dstuva, above--fate vs. free will. If, indeed, the witches are simply telling Macbeth what is ahead for him and he plays out what he is simply destined to do, can he be held as accountable for his actions? If, on the other hand, the witches are not agents of fate but simply toying with a Macbeth who has the free will to do what he wishes, Macbeth alone is to blame for anything which happens as a result of his actions. It really is less about the witches than about the issues of fate and free will; however, it's the witches who embody that argument. In our discussions, most want to believe they are more than agents of fate, living out a predetermined, preordained life; however, this is something with which every audience will wrestle.
A common theme of the play that is often discussed is evil. The evil presence in the play is most often attributed to the Witches, but different performances of Macbeth can present vastly different sorts of "witches."
In Shakespeare's day, witches were feared. There were "witch hunts" in which the Church sought to identify and eradicate the menace and blasphemous practice of witchcraft. The trouble with that was, much of what the Church would have considered blasphemous was merely traditional uses of medicinal herbs and other ancient practices handed down from pre-Christian times.
So, in different productions of the play, you might see very different types of witches -- witches that are meant to be magical in an evil and sinister way or witches that are more benign, more of a throwback to the earliest traditions of the British isles.
How the Witches are presented in any production has a huge effect on how the audience interprets the theme of evil in the play, and where they decide to lay the "blame" for the destructive mayhem that ensues in the play. Below I have provided some links to demonstrate some of the approaches that have been taken.
One such issue in Shakespeare's Macbeth that may be interpreted differently by different people is the amount of blame Macbeth and Lady Macbeth deserve for the assassination of King Duncan.
Both are extremely ambitious. Both want Macbeth on the throne. But while Lady Macbeth is single-minded in her desire to assassinate Duncan and secure her husband the throne, Macbeth is not so single-minded. Macbeth decides in Act 1 not to kill Duncan. But his wife berates him and manipulates him and causes him to change his mind. Lady Macbeth plans the assassination, and Macbeth goes along with it.
Thus, the issue arises over who is more responsible for the assassination.
Additionally, you can through fate into the mix: if the witches know and predict the future, does that mean that the future is set? To what degree is Macbeth guilty, if the witches know the future? To what extent does Macbeth have free will, when the witches predict the future and his wife berates him?
These are all issues that different people judge differently.