What reason does Macbeth give the Murderers for wanting Banquo killed?
Macbeth tells the murderers that he has his own reasons for wanting Banquo murdered, and he provides them with several reasons why they should do it before he challenges their masculinity (a quick study--he's learned from the master--oops mistress).
We aren't told precisely what Macbeth has done, but that he has done something can be inferred from his comments to the murderers, all of whom apparently have a grudge not against Banquo, but against Macbeth. Macbeth tells them that it was Banquo and not his "innocent self" that caused their undoing. In fact, he tells them that he has already informed them of exactly how and by what means the offensive deeds were done. According to Macbeth, Banquo is the one whose "heavy hand hath bowed [them] to the grave and beggared them forever" (by which we understand that Macbeth must have destroyed their lives and their livelihood so completely that they are all desperate men.
Manhood is an important theme in the play. Indeed, Lady Macbeth uses this as a way to prod Macbeth forward to kill the King. Macbeth plays this card with the murderers when he asks them if they can endure a man like Banquo, whose "heavy hand hath bow'd you to the grave / and beggar'd yours for ever." Their response: "We are men, my liege"(3.1.90-91). Macbeth then tells them: "Both of you /Know Banquo was your enemy," explaining he himself cannot do the dirty work of killing Banquo because of mutual friends, and for that reason "to [their] assistance" does he "make love." To prove their manhood, and because they have nothing to lose (see 3.1.108-114), they agree.
IN Act III scene I Macbeth tells elludes to a previous conversation he has had with them, yet which does not actually appear in the text itself. Based upon what Macbeth says in III.i, we are left to infer that he told the murderers that Banquo was their enemy who wished upon their death and sought to do them harm and ill will. The details of this are pretty scarce, but Macbeth asks the murderers if their patience will be enough to tolerate this, to which they say no. Later in the Act, an anonymous, carefully disguised third murderer joins in - this third murderer is probably there to make sure the murders follow through on the murders of Banquo and Fleance.