The witches in Macbeth are mischievous, and their prophecies are misleading even though they are true.
The witches in Macbeth manipulate him for the fun of it.
The first set of witches plays with Macbeth by telling him he will be king and Banquo’s sons will be king. This gives Lady Macbeth the idea to kill Duncan and put Macbeth in his place.
The three weird sisters seem to not have permission to do this, as Hecate admonishes them in Act 3 that they are “saucy and overbold” in trying to mess with Macbeth without her.
How did you dare
To trade and traffic with Macbeth
In riddles and affairs of death;(5)
And I, the mistress of your charms,
The close contriver of all harms,
Was never call'd to bear my part,
Or show the glory of our art? (Act 3, Scene 5)
Hecate is the head witch, and more powerful. Once Hecate gets involved, things get more serious. Her prophecies are more inventive and more specific. She guides Macbeth in his confidence while simultaneously telling him where the threats are.
Be lion-mettled, proud, and take no care
Who chafes, who frets, or where conspirers are:
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (Act 4, Scene 1)
Unfortunately (or fortunately), Macbeth misinterprets the prophecies and feels that he is indestructible. She provides him with a series of apparitions telling him the story of his future. It is accurate, but Macbeth cannot think past the metaphors and realize that he really is in danger.