The theme of "fair is foul and foul is fair" runs throughout Shakespeare's play, Macbeth.
Note the appearance of the three witches: first when they meet Macbeth on the heath after the battle with Norway has been won, and later when they conjure apparitions and prophecies for Macbeth. In Act One, scene three, Macbeth introduces the theme:
So foul and fair a day I have not seen. (39)
Next, the witches give Banquo and Macbeth predictions of what the future holds, but it is Macbeth (with his vaulting ambition—our tragic hero's flaw) who takes these words to heart and ultimately decides to murder his King, Duncan.
The theme applies here because the witches' predictions sound wonderful ("fair" or good), but they are evil—for their intent is to trip up Macbeth to the point that he will sacrifice his eternal soul in not only Duncan's death (for it was a mortal sin to kill a king), but for all the other horrible things he does to keep his place on Scotland's throne (and these things are "foul" or bad).
All hail, Macbeth! hail to thee, Thane of Glamis!
All hail, Macbeth! hail to thee, Thane of
All hail, Macbeth, that shalt be King hereafter! (50-53)
Once the prediction for Cawdor comes true, Macbeth believes that the last one—that he will be king—is guaranteed. Banquo warns Macbeth of the "fair is foul" theme:
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence— (132-136)
Banquo notes that evil "truths" ("fair") can lead to destruction ("foul"). Macbeth ignores the advice. The witches' second set of predictions simply solidify Macbeth's belief that he cannot be vanquished, even though the also decides to kill Macduff and his family—just to be sure.
"Fair is foul" is found with Banquo's death as Macbeth tries to explain it away, noting that it is dangerous to go out at night after dark. In truth, Macbeth ordered Banquo's murder, knowing that Banquo was so ethical that he would never be able to keep his knowledge of the witches' predictions (which would point to Macbeth's part in Duncan's death) a secret. Banquo is a good man ("fair"); he is murdered because of Macbeth's evil intent ("foul").
Fleance's death is an example of "fair is foul" because he (Banquo's son) is completely innocent ("fair"), and he flees for his life—while his father is being murdered. Macbeth tries to blame Fleance for his father's murder—this is "foul."
In Act Three, scene four, lines 5-7, Lennox, a lord in Macbeth's court, speaks to another lord with dripping sarcasm—explaining how Macbeth gives socially acceptable excuses that cover up the truth of what is really going on (and his lies seem "fair"), but the truth is that these murders are visited only upon those closest to Macbeth—or in his way ("foul").
Till the end, Macbeth believes the witches; facing Macduff, he tells his enemy he cannot hurt Macbeth—he is protected by the prophecies:
Thou losest labor. (V.viii.11)
I bear a charmed life, which must not yield
To one of woman born. (15-16)
Ultimately, Macbeth realizes what has happened. While the witches promised him a multitude of desirable things ("fair"), Macbeth realizes the witches' betrayal ("foul"):
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. (23-26)