Macbeth is saying that being king is not enough unless he remains king, and he says this right after he kills Duncan and before he kills Banquo.
Banquo and Macbeth were friends and fellow captains in Duncan’s army. All evidence points to the fact that they were friends, or at least that Banquo looked out for Macbeth’s interests and worried about him.
Banquo was there when Macbeth first heard the witches’ prophecies, so he knows what they are. He knows that Banquo might be suspicious, because he heard the witches tell him he’d be king before Duncan was killed. Banquo could ruin the whole plan.
’Tis much he dares,(55)
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valor
To act in safety. (Act 3, Scene 1)
But there’s another problem. The prophecy also said that Banquo’s sons would be king. This cannot be. Macbeth does not want a “fruitless crown.” He has to get rid of Banquo and his son. Macbeth decides to hire murderers to take out both Banquo and his son Fleance.
Macbeth goes from not wanting to commit one murder to committing several. He seems to get a taste for blood, but he realizes that he has not commit another murder to cover up the first one, and another, and another.