That, trusted home,
Might yet enkindle you unto the crown,
Besides the thane of Cawdor. 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.

In Act 1, scene 3, Banquo muses on the events of the last few minutes: just as the Witches predicted, Macbeth has been named Thane of Cawdor. Banquo notes that just because the Witches told the truth doesn’t mean that they’re not evil. Banquo understands far earlier than Macbeth that the Witches don’t necessarily have Macbeth’s best interests in mind, and their prophecies may turn out to be less positive than Macbeth believes.

Thou has it no king, Cawdor, Glamis, all.
As the weird women promised, and I fear
Thou played’st most foully for ‘t. Yet it was said
It should not stand in thy posterity,
But that myself should be the root and father
Of many kings. If there come truth from them –
As upon thee, Macbeth, their speeches shine –
Why, by the verities on thee made good,
May they not be my oracles as well,
And set me up in hope? But hush, no more.

In Act 3, scene 1, Banquo’s soliloquy reveals that he is suspicious of Macbeth, who, in becoming king, has achieved all that the Witches promised for him. Banquo senses that Macbeth engaged in foul play in order to make the Witches’ prophecy come true. While the idea that Macbeth may have murdered Duncan fills Banquo with fear, the thought also gives Banquo hope that what the Witches predicted for him will come true. He doesn’t realize that in order for his sons to become king, he will have to die.

O treachery! Fly, good Fleance, fly, fly, fly!
Thou may’st revenge – O slave!

These lines are Banquo’s dying words, as he is slaughtered by the murderers Macbeth has hired in Act 3, scene 3. In his dying breaths, Banquo urges his son, Fleance, to flee to safety, and charges him to someday revenge his father’s death. This sets the stage how the play will end, when Macbeth realizes that the Witches’ prophecy will come true, and Banquo’s children will rule Scotland.