Lady Macbeth and Banquo both die because of the transformation in Macbeth. Both were fair, and become foul—either to themselves or to Macbeth.
The witches tell us that what is good is bad and what is bad is good. In the case of Lady Macbeth, this is definitely true. She thinks that Macbeth being king is a great idea, and even encourages him to kill Duncan so that he can be. Then, he ends up killing several more people and she begins to wonder if she has created a monster.
Nought'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. (Act 3, Scene 2, p. 45)
Lady Macbeth thinks that he should just sit back and enjoy what he has, but Macbeth is not secure. He tells her that they have not gotten rid of the threat, and she begins to worry. By Act 5, she is full of regret for what she has done. She ends up killing herself because of the consequences of fair being foul.
The Thane of Fife had a wife; where is she
now? What, will these hands ne'er be clean? No more o’
that, my lord, no more o’ that. You mar all with this(40)
starting. (Act 5, Scene 1, p. 77)
The guilt of what he has done, and her part in it, destroys her.
Banquo was Macbeth’s trusted friend. He was there when the prophecy was given. Yet because he was named in the prophecy, and the witches said his sons would be king, Macbeth suspects him. He tries to warn Macbeth not to listen to the witches, but Macbeth is too far gone.
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's(135)
In deepest consequence—
Cousins, a word, I pray you. (Act 1, Scene 3, p. 15)
In the end, Macbeth decides that Banquo, once fair, is now foul. He is nothing but a threat to be eliminated at all costs.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’ (Act 3, Scene 1, p. 42)
Before long, Macbeth kills Banquo. He proves again that fair is foul, since the two were friends but he is willing to kill his friend to secure his ambitions.