Banquo’s murder suggests that even evildoers experience guilt and regret.
When Macbeth first decided to kill Banquo, it was because the witches told him that Banquo’s sons would be king. He needed Banquo out of the way. The third witch says to Banquo “Thou shalt get kings, though thou be none” (Act 1, Scene 3). It is because of this that Banquo must die. It is also partly because Banquo is getting suspicious.
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't … (Act 3, Scene 1)
Banquo knows too much. He is the only one other than Macbeth to witness the witches’ prophecies. He is able to put two and two together. He knows as soon as Duncan is murdered that Macbeth is the one to do it. Between that and the fact that Banquo’s sons are supposedly the heirs to the throne, and not Macbeth’s, Banquo has to die.
Unfortunately, that is not the end of Banquo for Macbeth. Macbeth has several lords over to dine, and has a strange surprise. When he tries to sit at his place at the table, he finds it “full” of Banquo’s ghost!
Here had we now our country's honor roof'd,
Were the graced person of our Banquo present;
Who may I rather challenge for unkindness(50)
Than pity for mischance! (Act 3, Scene 4)
This is a sure sign of Macbeth’s guilty conscience. He’s cracking up! His madness is a sign that he knows what did was wrong, murdering his friend (and the murder was very recent), and he can’t quite handle it. This guilt and madness was seen earlier in his unsettled demeanor after killing Duncan, and will be seen again as he comes more and more unglued.
Macbeth will not hold onto his kingdom for long, because he cannot hold himself together due to the guilt at how he got it. This shows that even an evildoer like Macbeth can have a conscience. We see this again in Lady Macbeth. She is the one who convinced Macbeth to kill Duncan, and her conscience gets the best of her too. Guilt drives her mad, and she kills herself.