The three witches like to cause trouble. This is demonstrated at the beginning of Act I, Scene 3. The Second Witch tells the First Witch that she has been killing swine. Evidently, this is just one of the things the Second Witch enjoys doing. The First Witch tells the other two that she intends to get revenge on a sailor's wife who refused to give her chestnuts.
Her husband's to Aleppo gone, master o’ the Tiger;
But in a sieve I'll thither sail,
And, like a rat without a tail,
I'll do, I'll do, and I'll do.
The other two witches are eager to help their sister. Both promise to give her winds to enable her to sail towards Aleppo in her sieve. They listen with obvious pleasure as the First Witch tells them how she plans to torment the husband of the girl who refused to share her chestnuts.
I myself have all the other,
And the very ports they blow,
All the quarters that they know
I’ the shipman's card.
I will drain him dry as hay:
Sleep shall neither night nor day
Hang upon his penthouse lid;
He shall live a man forbid.
Weary se'n nights nine times nine
Shall he dwindle, peak, and pine;
Though his bark cannot be lost,
Yet it shall be tempest-tost.
This is quite a cruel revenge to be taking over a few chestnuts. If the three witches spend their time causing trouble and take such delight in doing so, then it seems possible that their sole intention in meeting Macbeth and Banquo in this scene is to cause the two men trouble, and, further, that none of the troubles would have occurred if they had not made their predictions. In that case, it may be that these wicked creatures are devious enough to invent lies calculated to cause the maximum trouble for both men.
They tell Macbeth that he will be king of Scotland, and they tell Banquo that he will be the sire of future Scottish kings, although he will not be a king himself. Both prophecies may be utter lies. Macbeth has been thinking of murdering Duncan in order to usurp him. The witches' prophecy only encourages him to give the idea deeper consideration. He writes a letter to his wife in which he tells her about his meeting with the witches. Lady Macbeth reads the letter aloud at the beginning of Act I, Scene 5.
They met me in the day of success, and I have learned by the perfectest report, they have more in them than mortal knowledge. When I burned in desire to question them further, they made themselves air, into which they vanished. Whiles I stood rapt in the wonder of it, came missives from the King, who all-hailed me "Thane of Cawdor"; by which title, before, these weird sisters saluted me, and referred me to the coming on of time with "Hail, King that shalt be!" This have I thought good to deliver thee, my dearest partner of greatness, that thou mightst not lose the dues of rejoicing, by being ignorant of what greatness is promised thee. Lay it to thy heart, and farewell.
This letter spreads the witches' possibly false information further. Lady Macbeth is, as she says, "transported." She will become the most important instrument in carrying out the witches' destructive plans. Macbeth kills Duncan and his two guards. Duncan's two sons flee for their lives, enabling Macbeth to blame Duncan's murder on them and become king of Scotland, as predicted.
Then, the hypothetically false predictions of the three witches work further troubles. In fact, it might be said that all the troubles in the play emanate from the witches' evil minds. Macbeth and Banquo now hate and fear each other. Macbeth arranges to have Banquo and his son Fleance ambushed. Banquo is killed, but Fleance escapes, so there is still the possibility that the prophecy will come true that Banquo's descendants will be kings of Scotland.
Macbeth becomes a tyrannical ruler. Evidently, he finds it necessary to rule by terror because everyone hates him. They know he murdered Duncan and had Banquo murdered. The whole country falls into chaos. Is this another result of the witches' wicked planning? The English king sends an army to Scotland, and Macduff kills Macbeth in a death duel. The troubles created by the Weird Sisters finally come to an end. Macbeth realizes he has been tricked.
And be these juggling fiends no more believed,
That palter with us in a double sense,
That keep the word of promise to our ear,
And break it to our hope.
It might be maintained that the three witches had no other motive from the very beginning than to cause troubles—and they certainly succeeded. The audience is left guessing as to whether the witches acted independently to lie and manipulate Macbeth or if they truly were reporting inevitable prophecies.