Macbeth is attempting to goad the two men into accepting the mission of murdering Banquo and Fleance by asking them to think about the kind of men they are. He makes an analogy by listing types of different dogs:
. . . hounds and greyhounds, mongrels, spaniels, curs,Shoughs, water-rugs, and demi-wolves are cleptAll by the name of dogs
In this scene, Macbeth uses rhetoric in much the same way Lady Macbeth did in Act I, scene vii. Macbeth attacks the manhood of the murderers, comparing them to different kinds of dogs. He wants to get them so angry that they will have no qualms about killing Banquo and Fleance. The first murderer states that both of the murderers are men, indicating they will do what is necessary. Macbeth's response to them is when he makes the comparison of the men to dogs. Macbeth says men are just like dogs. Just as there are different breeds of dogs with different qualities, there are different kinds of men. He wants to know if the murderers are the kind of men who can do whatever is necessary in order to get rid of both Banquo and Fleance.