In Act 3, Scene 2, Macbeth tells his wife:
O, full of scorpions is my mind, dear wife!
Thou know'st that Banquo and his Fleance lives.
She is equally concerned about this fact. She has expressed this torment to herself just before her husband enters.
Naught's had, all's spent,
Where our desire is got without content.
'Tis safer to be that which we destroy
Than by destruction dwell in doubtful joy.
She does not want Banquo to be sire of a whole line of Scottish kings any more than does her husband. She tells him:
But in them nature's copy's not eterne.
This seems to be the same as saying that if the father and his only son could be eliminated it would negate the witches prophecy. Here both Macbeth and his wife are conspiring against Fate itself. How could the witches' prophecies for Macbeth have come true without the prophecy concerning Banquo and his heirs also coming true? Macbeth believes it is possible because he wants to believe it. He tells his wife:
There's comfort yet; they are assailable.
Then be thou jocund.
Macbeth has already arranged to have Banquo and Fleance assaulted and murdered outside the castle when they return from their ride. All Macbeth and his wife have to do is to be "jocund," that is, cheerful and carefree, during the evening at the banquet until the bad news is brought to them. They will be surrounded by witnesses who will see that they were not involved in the killing.
Hmm. It sounds like somebody's channeling the witches. When Macbeth talks about his plans for the murder of Banquo and Fleance, he starts sound a lot like the weird sisters.
There's comfort yet; they are assailable;
Then be thou jocund: ere the bat hath flown
His cloister'd flight, ere to black Hecate's summons
The shard-borne beetle with his drowsy hums
Hath rung night's yawning peal, there shall be done
A deed of dreadful note. (3.2.4)
HECATE Have I not reason, beldams as you are,
Saucy and overbold?
How did you dare
To trade and traffic with Macbeth
In riddles and affairs of death;
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?
And, which is worse, all you have done
Hath been but for a wayward son,
Spiteful and wrathful, who, as others do,
Loves for his own ends, not for you. (3.5.1)
Even witches have to follow orders. In a way, the witches' disobedience seems like a parallel to the way Macbeth, "the wayward son," is insubordinate to King Duncan. The "supernatural" still has rules and hierarchy; what Macbeth is doing is unnatural, inverting the natural order of king and lord.