In Act III, scene I, how does Macbeth manipulate the murderers' minds?

This scene when Macbeth hires the two murders to do his dirty work for him is very interesting when we consider what is shows about the development of Macbeth's character. Macbeth no longer meets his enemies and threats directly, but gets others to do his work for him - something that displays his own political strength but also his moral weakness. Now he must accomplish his crimes from a distance: "something [distant] from the palace”. It is also ironic how Shakespeare compares the guilt of Macbeth to the idea of murder with the murderers' very pragmatic understanding of the crime. 

Macbeth has to persuade the murderers to carry out his will. It is clear that Macbeth knows the murderers from before. His speech is aimed at creating a hatred of Banquo to impel them to murder him: “Know that it was he …,” “This I made good to you in our last conference,” “Do you find your patience so predominant in your nature that you can let this go?” The aim of his speech is to ensure that the murderers are without guilt or moral scruples, as if they have sympathy they might not kill Banquo according to Macbeth's wishes. Empathy on the part of the murderers would be a natural human response, but this is precisely what Macbeth tries to prevent. Thus when the First Murderer says to Macbeth "We are men, my liege,” Macbeth interrupts him and, by using a series of metaphors, lowers the level of humanity of these murderers to the level of beasts: “Ay, in the catalogue ye go for men, / As hounds and greyhounds, mongrels, spaniels, curs / … and demi-wolves are clept [called] / All by the name of dogs". Hardly very complimentary, but Macbeth is hell-bent on achieving his purpose - ensuring that they do their job effectively without guilt creeping in.

Macbeth does flatter the murderers by hinting that their success in this job will raise them above the low social standing they have at present, his tone is definitely ironic and indicates he thinks of them as nothing more than tools to be used or just like the beasts he has just compared them too. Of course, the irony is that this opinion reflects back on Macbeth and his own character.