They can torment him but not change his fate.
Though his bark cannot be lost
Yet it shall be tempest tossed
The audience are also aware that the second greeting is not a prediction as the order to greet Macbeth with the title 'Thane of Cawdor' was given by Duncan in the previous scene. There can be little doubt that the witches are exploiting the situation for their own evil ends and are using "honest trifles" to win Macbeth to harm, but given the limited nature of their powers, it is hard to say that they are responsible for Macbeth's later actions.
Like the witches, Lady Macbeth is crucial to the actual accomplishment of Macbeth's crime. Without her, Macbeth would not have carried out the murder in the first place - "we shall go no further in this business" - and without her timely interventions in gilding the groom's faces with blood and conveniently fainting when Macduff's questions become too insistent, it is unlikely that he would have got away with it. She seems to be just as ambitious as her husband and the plan to kill Duncan is largely hers. She overcomes Macbeth's scruples by both encouragement and scorn:
Macbeth Pr'ythee, peace!
I dare do all that may become a man;
Who dares do more is none.
Lady Macbeth What beast was't, then,
That made you break this enterprise to me?
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man.
Also like the witches, Lady Macbeth calls explicitly on the power of evil and asks to be filled with "direst cruelty" in order to overcome the "milk o' human kindness" that is too prevalent in Macbeth's nature. She is not inhuman, however, and refrains from killing Duncan herself because he resembles her father. In fact, her humanity reasserts itself after the murder and it is she, not Macbeth, who descends into guilt-induced madness and despair, taking no further part in Macbeth's crimes.
Macbeth Be innocent of the knowledge, dearest chuck.
Lady Macbeth is the catalyst that ensures the witches' evil desires become reality. She appears to think that the murder of Duncan is the only obstacle on the road to power and that the deed will have no consequences
A little water clears us of this deed.
She pushed her husband towards killing the king but ultimately it is Macbeth who does the deed. All school children are aware that there is a vast difference between encouraging someone to do something and doing it yourself: 'and if your friend told you to jump off a cliff would you do that as well?' teachers have asked rhetorically over generations, knowing that the answer is 'no'. Lady Macbeth cannot be held responsible for her husband's crimes although in a modern court she might be tried as an "accessory before and after the fact".
Macbeth begins the play as a successful warrior with a bright future. It is even possible that he might be named king as Duncan's successor since being the king's son was less important in those times than being a strong leader and warrior. Such a course of events could have been implicit in the witches' greetings but almost immediately Macbeth thinks of a less legitimate path to the throne.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair,
And make my seated heart knock at my ribs,
Against the use of nature?
In his more optimistic moments Macbeth thinks that
If chance will have me king, why, chance may crown me
Without my stir
but once Malcolm is named as Duncan's heir the honest route to the crown is blocked and Macbeth does indeed yield to the suggestions that came to him at the very first. The fact that Macbeth agonises over his crime, has to be persuaded to commit it by his wife, and has guilt-ridden hallucinations as he is about to commit it, cannot excuse the fact that he did commit it and that it was his idea in the first place.
The witches' greetings go right to the heart of Macbeth's ambitious nature. He clearly enjoys power and success in battle and wants to rise to be the ultimate power in his society. In gaining the crown, Macbeth loses security and peace of mind.
I heard a voice cry, "Sleep no more!
Macbeth does murder sleep,"-- the innocent sleep;
Sleep that knits up the ravell'd sleave of care,
The death of each day's life, sore labour's bath,
Balm of hurt minds, great nature's second course,
Chief nourisher in life's feast.
This loss of his clean conscience begins the internal transformation of Macbeth from a man of honour into a bullying tyrant. Similarly, the need to secure his position against external threats leads to his murder of Banquo and Macduff's family. The once great warrior who his wife thought "too full o' the milk of human kindness" becomes desensitised to the death and suffering he brings about and he is incapable of escaping from the bloody conclusion to his course of action.
I am in blood
Step't in so far that, should I wade no more,
Returning were as tedious as go o'er
This, then, is the tragedy of Macbeth. A single incident that plays upon the worst part of his character leads inevitably to his death. A less ambitious man would not have been tempted, a weaker man would not have dared, but because of his circumstances and his character, Macbeth falls. Aristotle said that one of the signs of a tragedy was that it must terrify the audience. Like Macbeth at the beginning of the play, most members of the audience are likely to have a clear conscience, but what if something happens that reveals your true, ugly nature? Shakespeare chose to make the starting event an encounter with witches, partly to flatter the witch-hating James I and partly to introduce an exciting flavour of the supernatural into the play. But, exciting as the scenes with the witches and Hecate are, they are not at the heart of the tragedy. The witches' powers are limited, Lady Macbeth is only a helper: it is Macbeth's own weaknesses that bring him down.
