In Shakespeare's Macbeth, after Macbeth kills Duncan and takes the throne, Macbeth is aware that in order to guarantee his place as King of Scotland, he must kill Banquo. Even though the two men are close, their moral compasses are nothing alike. Though Lady Macbeth fears her husband is not brave enough to take the quickest way to become king, practice makes perfect, and Macbeth now knows that Banquo must die. He is the only person who was present to hear the witches' first set of predictions. They also gave Banquo predictions, but Banquo did not act on them—as Banquo suspects Macbeth has done. Banquo privately considers Macbeth:
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't… (III.i.1-3)
Macbeth asks Banquo how far out he will ride, in order to know about what time he will return, which he will the murderers he has hired—having convinced them that their misery in life is Banquo's fault. (Ironically, Macbeth "orders" Banquo not to miss the feast, even knowing that he will be dead. However, Banquo is true to his word, showing up as a ghost and reducing Macbeth to a sniveling wretch as opposed to the powerful man described earlier in the play.)
...Is't far you ride?
As far, my lord, as will fill up the time
’Twixt this and supper. Go not my horse the better,
I must become a borrower of the night
For a dark hour or twain.
Fail not our feast.
My lord, I will not. (III.i.26-32)
Macbeth realizes that he has risked all to take the throne. After everything he has done, even sacrificing his immortal soul (because the Elizabethans believed it was a mortal sin to kill a king), Macbeth realizes that as long as Banquo knows of the predictions, he will have second thoughts about his Macbeth. Banquo has already told Macbeth that he will support him as long as he is not asked to do anything that would compromise his integrity. Macbeth knows that Banquo cannot be turned "to the dark side." The only solution is to kill his friend. And because the witches told Banquo that his "issue" (descendants) would be kings, Macbeth knows that unless he kills Fleance, too, the throne will move into Banquo's family, as Macbeth has no children. He says that in this case, he has sold his soul ("mine eternal jewel") for someone other than his own son.
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...There is none but he
Whose being I do fear...He chid the sisters [and]...
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown
...to be wrench'd with an unlineal hand,
No son of mine succeeding. If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered,
...Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings! (52-74)
This series of events shows us how Macbeth has lost all touch with the valiant man he was at the play's beginning, compared to Banquo will not be swayed to follow Macbeth, regardless of the reward—if Macbeth is acting in any way unsuitable for a king. Macbeth's descent into murder and madness is evident and he continues to kill in order to maintain his place.