Yeah, you are going to get a bunch of answers for this one, so be ready to start assessing validity from different points of view. Part of this play's greatness is that it draws some very strong lines between fatalism (the belief that something is destined to happen) and free will (the believe that we as human beings are purely responsible for what will happen). In answering if the witches are the cause of MacBeth's downfall, you have to answer if you are a fatalist of a free will person. Shakespeare creates the witches to be a source of question and analysis. Part of this is reflective of us, the readers/ audience. It seems to me that a fatalist would believe that once MacBeth started to enter this particular realm, he lost some of his control over the exactitude of his destiny, as higher levels of power began to assert their influence. An advocate of free will would probably still say that it was MacBeth's choice to solicit help/ advice/ counsel from the witches and he was the agent of his actions. (It is interesting that your question is about the witches and not about Lady Macbeth.)
The answer to the question of culpability might also reside in how a person traces the downfall of MacBeth. It seems that when one starts to analyze the causes for his downfall, one starts to walk down a slippery slope in trying to discern MacBeth's relationship to the witches' prophecies. Does he use them to justify his actions or does he believe he is simply a vessel for the witches' actions/ prophecies? Is he in control or does he seem to surrender control to these "higher powers"? In asking how responsible he is for his downfall, I think that there are some gray zones, but some definites that have to be addressed. Does Shakespeare create MacBeth to be the author of his own actions? Are there some definite actions for which MacBeth has to take sole responsibility? Are there some influences on him that diminish his capacity to assume full responsibility? Might Shakespeare be suggesting that there is some type of balance between human action and circumstances around it? Could MacBeth's downfall be a tragic convergence (coming together) of malevolent forces and human misdeeds? There is not going to be a simple answer to this question or the question of MacBeth's downfall, proving the merit of the work. The answer to such questions will be based on you, the reader, and your dispositions on how you view the nature of human action, and in particular, MacBeth. The person you are and your belief system will determine your answers because the text can go either way on it, as justifications abound on each side.
As I said, you will get a great many responses here and your best bet will be to assess the validity of the opinions presented and your own view of it, going with what you think becomes "the best answer." This one will not be easy, though. I think that's what makes both the question and the work powerfully compelling.
In Macbeth, the presence of the witches provides the main character, Macbeth, with incentive, or motivation, which awakens in him, his deeply held desire to be king. The witches are not responsible for Macbeth's downfall, he chooses to kill the king because he suffers from unchecked ambition, which is at the heart of his decision.
However, I would agree that Lady Macbeth had more to do with Macbeth's decision than the witches. She becomes inspired with the idea to kill the king, once she receives the news from her husband about his elevation to Thane of Cawdor.
The witches conspire to influence Macbeth, in Act I, they make a decision.
"First Witch. Where the place?
Sec. Witch. Upon the heath.
Third Witch. There to meet with Macbeth.
First Witch. I come, Graymalkin!
Sec. Witch. Paddock calls.
Third Witch. Anon.
All. Fair is foul, and foul is fair:
Hover through the fog and filthy air. [Exeunt." (Shakespeare)
After Banquo and Macbeth encounter the witches and the prophecy is given to both of them, Macbeth does not really believe what the three witches have told him. But then he receives news the shocking news that he will be made Thane of Cawdor, just as the witches said.
"Macb. [Aside.] If chance will have me king, why,
chance may crown me,
Without my stir.
Ban. New honours come upon him,
Like our strange garments, cleave not to their
But with the aid of use.
Macb. [Aside.] Come what come may,
Time and the hour runs through the roughest day.
Ban. Worthy Macbeth, we stay upon your
In this exhange, Banquo and Macbeth discuss the fact that the prophecy, at least part of it has come true. Macbeth states that if he is to be king, it would happen without him doing anything at all. Banquo believes that Macbeth is receiving the benefit of hard work, that he is worthy. So at this point, Macbeth does not see any reason to kill the king.
Later in the play, when Macbeth tells his wife of his new honor and that the King is coming to their home for dinner, she can't believe her good fortune. It is her idea to kill the king.
"Lady M. Give him tending;
He brings great news—[Exit Messenger.] The
raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. Come, you spirits
That tend on mortal thoughts! unsex me here,
And fill me from the crown to the toe top full
Of direst cruelty; make thick my blood,
Stop up the access and passage to remorse," (Shakespeare)
In this speech, Lady Macbeth longs to be made a man so that she can carry out the deed of killing the king. She comes up with the plot to murder Duncan, the only reason she does not do the killing herself is because when she enters the king's bed chamber, she says that the sleeping king looks too much like her father.
Macbeth is influenced more by his wife than the witches, it is Lady Macbeth who, along with Macbeth, himself, who bear the most responsibility for the death of the king and Macbeth's ultimate downfall.
"Some critics maintain that responsibility for the deaths of Duncan and Banquo rests solely with Macbeth, whose own ambition and nature are the cause of his deeds."
"Others cite Macbeth's reluctance prior to Duncan's murder and argue that Lady Macbeth goads her husband into the action. Lady Macbeth does, however, set the time and the place of Duncan's murder, claims that she would kill a baby at her breast to honor a vow, and argues that when Macbeth first conceived of killing Duncan, then he was a man."
I will answer the first question, Are the witches responsible for Macbeth's action?
As a teacher, I have always taught my students that the answer to this question is NO. Macbeth makes decisions on his own. Yes, the witches plant the seeds in his mind. But it is Macbeth who makes the decision to take that first step, of killing King Duncan. It is Macbeth who makes all the decisions from then on. At first, he needs Lady Macbeth to gorde him and tell him to man up and do what needs to be done to get what he wants. After that first murder, Macbeth has control. He is the one who makes the decisions. he is the one with the blood on his hands. Just as every man is given a conscience and free will by whatever Greater Being, or God, she/he believes in, Macbeth uses his free will to make the decisions he makes. The witches were only the tools of his tempation.
I think it is not possible to say that the witches are entirely responsible for the downfall of Macbeth.
Firstly, it is possible to see that the witches' prophecies are not 100% true, and they do not always work out. I believe that there is a reason why the prediction given about Banquo that, "Thou shalt get kings" in Act One Scene 3 was never fulfilled or addressed later on in the play was to show that the prophecies made by the witches did not determine the fate of the characters. Instead, as Banquo doubted whether the witches' prophecies can really determine the prophecy, "What, can the devil speak true?" (Act One Scene 3) and persisted in repressing his ambitious side, and pleaded that "Merciful Powers, restrain in me the cursed thoughts that nature gives way in to respose", Banquo did not act on the prophecy given.
However, in contrast, we can see that even if without the Banquo example showing that the prophecy might not be 100% true, the witches are not entirely responsible for the downfall of Macbeth. While it is true that the witches might have possibly stirred up Macbeth's ambition that "the greatest is behind", that Macbeth might become king, Macbeth, and his wife Lady Macbeth, were the ones that formulated the plan to murder King Duncan. Macbeth was the one that thought out and imagined of the plan to murder and of "horrible imaginings". While Macbeth seemed to be initially willing to let and could have completely allowed "chance crown me without my stir", it was Macbeth's own impatience and insecurity, especially after Duncan's declaration of Malcolm as the heir to the throne (as show Macbeth exclaimed "The Prince of Cumberland!"), that led him to think of the plot to kill to address his "black and deep desires".
The witches, however, still served an important role for the downfall of Macbeth. Most crucially, they are "equivocating fiends", they "win us with honest trifles". The witches seem to have some control over the destinies of the characters in the play, however, they also seem to desire to manipulate Macbeth into committing immoral and evil acts by convincing him of that the prophecies were true. It can be seen in Act One Scene 3, in which almost immediately after the witches hailed Macbeth as the Thane of Cawdor and King, and then disappeared, Ross and Angus met with Macbeth and Banquo and hailed Macbeth as "the Thane of Cawdor". This certainly convinced Macbeth of the truth of the prophecies as it gave Macbeth "earnests of success, commencing in a truth", refuting the doubts that Macbeth had about the prophecy, though Macbeth can be seen as inclining to believe it ("went it not so?"). However, whilst it is possible to point out that Macbeth's own ambitious nature, in contrast with Banquo's, led him to believe deeply in the prophecies and act on it, the fact that the first part of the prophecy was fulfilled by the prophecy of the witches, or rather the witches somehow allowed it to happen was a significant influence on Macbeth's belief in the prophecy and hence influencing his decision to murder Duncan (moral and mental downfall?). In addition, the witches appearances in Act 4 Scene 1 and their spells that made apparitions appear, apparitions that were "half-truths", can be seen as attempts by the witches to persuade Macbeth not to give up and to resist the attacks by Malcolm's forces as "no man born of woman" will harm him and "until Birnam Wood comes to Dunsinane" will he be vanquished.