The scene in which Macbeth consults with the murderers, Act III scene 1, is actually a very important scene in terms of the moral decline of Macbeth. Having allowed his wife to do the dirty work and the manipulation before, here Macbeth shows that he is capable of doing such acts by himself without his wife's knowledge. His conversation with the murderers is an excellent example of how one man can manipulate and distort the truth for his own purposes to persuade others to do what he wants them to do. It is clear that Macbeth has met with these murderers before, and he has sown the idea that it is Banquo who is responsible for their misfortunes, as is made clear in his first speech to them:
Have you consider'd of my speeches?--know
That it was he, in the times past, which held you
So under fortunes, which you thought had been
Our innocent self?
We don't know the truth, but Macbeth may have actually been the one who disadvantaged these men and now he has seized this opportunity to conveniently blame it upon Banquo to raise the ire of the murderers against him and manipulate them to achieve his purpose.