Before Macbeth has "Done the Deed" which we know to be killing Duncan, he is fairly loyal and wary of killing his king. He is a great leader and soldier who is greatly admired and respected by Duncan. It isn't until the witches proclaim that Macbeth will one day be king does he begin to think of his future. Lady Macbeth runs with this prophecy and decides that they must act to make their becoming royalty happen sooner.
As a subject of King Duncan and a host to him, he has certain expectations placed upon him by society. Lady Macbeth has to do a lot of convincing to make sure Macbeth goes through with their plan. Before killing Duncan, Macbeth feels so guilty for considering it, he hallucinates "Is this a dagger which I see before me, The handle toward my hand?" (II.i.) It isn't until he hears Lady Macbeth's signal that he finally gets the courage together to go into Duncan's chamber to do "the deed."
Macbeth comes back with the murder weapons in his hands, and Lady Macbeth must clean up his mess and put them back in the guard's chambers to frame those men. Macbeth is jumpy and paranoid hearing noises that Lady Macbeth shrugs off as just being the owls and natural world.
Once the night is over and the other nobles arrive in the morning for the king, Macbeth has completely transformed from a loyal servant to a paranoid criminal finally to a ruthless killer. Once he has disposed of the guards, he takes control of the situation. No longer does he visibly question whether or not what he had done was moral. The sons of Duncan take off for other countries out of fear that whoever plotted the death of their father would come after them next, and Macbeth is made king in their suspicious absence.
The roles reverse in this scene and Lady Macbeth faints upon hearing of Macbeth's bloody murder of the guards. He claims it was out of anger and rage, but Lady Macbeth has a very intense reaction to this. Throughout the rest of the novel, Lady Macbeth is kept out of the plotting as Macbeth plans the murders of the rest of the men who pose a threat to him: MacDuff and MacDuff's family and Banquo and Banquo's son, Fleance.