In Act Three, scene one, of Shakespeare's Macbeth, Macbeth speaks to the murderers to convince them to kill Banquo. The first reason is because Banquo has oppressed them—prevented them from having a better life (or so Macbeth says—it's actually a lie).
That it was he, in the times past, which held you
So under fortune, which you thought had been
Our innocent self? (81-84)
Next he gives the men specific examples of the things Banquo has allegedly done to harm them:
This I made good to you
In our last conference, pass'd in probation with you
How you were borne in hand, how cross'd, the instruments,
Who wrought with them... (84-87)
Macbeth tells them that Banquo went out of his way to "cross" them, and tells them that Banquo has and has used false (we infer) documents against them.
Then Macbeth appeals to their egos: he questions their patience: how long will they stand by and let Banquo harm them? Are they such good "Christians" that they would pray for Banquo and his family? And he reminds them that this same man has not only led these men to the brink of death, but also their families into abject poverty.
Do you find
Your patience so predominant in your nature,
That you can let this go? Are you so gospell'd,
To pray for this good man and for his issue,
Whose heavy hand hath bow'd you to the grave
And beggar'd yours for ever? (92-97)
One of the men shows his pride, insisting that they are men, not cowards or weaklings. Macbeth picks up on this and says that literally they are men, but there are men who will talk about doing and those who will take a stand and defy one who does them harm; a real man (he infers) is a man who takes action—even if that action is murder...for Macbeth has inferred, too, that what they would do would not only be for themselves, but also for their families.
Macbeth then encourages the men with news of a plan that will easily see to the end of Banquo and remove him from their lives, unable to harm them further.
Next, Macbeth assures them that if they do this for him, he will be indebted to them. Doing a favor for the King is an honor, a privilege:
And I will put that business in your bosoms
Whose execution takes your enemy off,
Grapples you to the heart and love of us... (111-113)
Finally, Macbeth tells them that he would commit the murder himself, but he cannot afford to offend those who are loyal to Banquo. This also gives the men a sense that they are helping the King, doing what he himself is unable to do.
Macbeth tells all of these lies because he fears Banquo's honesty—that Banquo will eventually expose Macbeth as a murderer, for Banquo is a deeply ethical man, who was completely devoted to Duncan. Macbeth also cannot bear the fact that while he has no sons to whom he can pass on his title, Banquo will "father a line of kings," and this eats away at Macbeth's already deteriorating mind.