In the beginning, Macbeth is fighting fiercely for King Duncan. In Act One, Scene Four, when King Duncan honors him, Macbeth confesses his loyalty to the King of Scotland:
In doing the service and the loyalty I owe you,
I am well paid. Your highness' role as King
Is to receive our duties. and our duties
Are to your throne and state, children and servants,
Who only do what they should, by doing everything
Loyal to your love and honor.
While Macbeth seems to be genuine in his comments to King Duncan, the witches have planted the desire in Macbeth to become king. Macbeth begins to dwell upon the idea of being king. He writes a letter to his wife expressing how he now "[burns] in desire" to know more about the prophecy. While Macbeth is curious, he does change his mind about the murder of King Duncan. He does seem to have second thoughts. He admits that King Duncan has honored him, and he decides to proceed no further in the business of murdering King Duncan. In Act One, Scene seven, Macbeth informs Lady Macbeth of his decision to not kill King Duncan:
We will proceed no further in this business.
He has recently honored me...
Clearly, Macbeth has changed his mind. He has a change of heart. Obviously, it is not something he really desires to do. He is human after all.
Lady Macbeth is furious with his decision. She uses manipulation and control to change Macbeth's mind. She admits that she would kill Duncan herself, but he resembles her father sleeping. She challenges Macbeth's manhood, trying to humiliate him. She asks if he is afraid:
Are you afraid to be the same man in reality as the one you wish to be? Would you have the crown which you believe to be the ornament of life, and yet live like a coward in your own self-esteem?
Lady Macbeth is heartless. She does not care about King Duncan. She is determined to get Macbeth to follow through with the murder. She is very influential and finally she convinces Macbeth to follow through with the murder. He gives in to her wishes though calling it terrible:
I’m convinced, and I commit every part of my body to this terrible event.
Here we see a man who had changed his mind about murdering King Duncan. He appeared to have a heart, but after Lady Macbeth questions his manhood, he gives in to her wishes.
Now we have a mixture of opposing elements. Obviously, Macbeth is not the type to be afraid of killing someone. He uses his sword daily to kill the enemy. He is quite brave. So why does he change his mind about killing Duncan when his wife challenges his manhood?Surely he has nothing to prove in terms of bravery. He is a fierce soldier.
At the same time, Macbeth is human and his wife admits that he is too kind to kill Duncan. He admits that King Duncan has honored him. Previously, Macbeth shares his loyalty to King Duncan.
Now, he allows his wife to change his mind. Lady Macbeth has a strong drive. She is excellent at manipulating and controlling Macbeth. She admits she would commit the murder herself. She is dangerous. She is convincing. She is the reason Macbeth changes his mind and commits the murder. Lady Macbeth is Macbeth's weakness.
In Shakespeare's Macbeth, the roles of Macbeth and Lady Macbeth at first are very different but together, they are able to bring about Macbeth's success in taking the throne. However, by the end of the play, it is as if husband and wife have exchanged roles and are now what the other was when the play began. By the time Macbeth is ready to kill Banquo, he has begun to pull away from Lady Macbeth, making his plans without her knowledge, and it is here that their "separation" becomes apparent.
When Macbeth receives the witches' predictions, he sends word first to his wife. Her excitement is seemingly greater than her husband's, and she begins to make plans before Macbeth or Duncan (the King) even arrive at Inverness. When Macbeth starts to question the plans to murder Duncan, thinking they should wait a while, Lady Macbeth insults his manhood, calling him all kinds of names—showing a treachery that Macbeth notes is more manly than one might expect from a woman. He says to his wife:
Bring forth men-children only;
For thy undaunted mettle should compose
Nothing but males. (I.vii.81-83)
After his murder of the King, Macbeth returns hysterical. He has brought the murder weapons with him and refuses to take them back. Lady Macbeth does so, and chides her husband for his foolish fears, saying that she has no qualms about doing so, and even rubs blood from the King's dead body onto the guards who they will blame for the murder.
By Act Three, scene two, Macbeth is "going solo." He hints to his wife that he plans to take care of Banquo, but wants her to praise him after the fact, and shares no details with her:MACBETH:
Be innocent of the knowledge, dearest chuck,
Till thou applaud the deed. (50-51)
By the time Macbeth seeks out the witches for a second set of predictions, Banquo is dead, and soon Macduff's family will be slaughtered on Macbeth's order. He has lost his fear of killing, but near the end of the play, we learn that Lady Macbeth has started to lose her mind. She relives the murder of Duncan and is even haunted by the murders her husband has committed, speaking of the details while she sleep walks. She has become very fragile and ultimately takes her own life, while Macbeth is fearless in all he does. He is so "steeped in blood," that he sees no point in stopping, for he cannot turn back time, and nothing will purge him of the guilt of his misdeeds.
While at the beginning Macbeth has a weak-stomach for murder and his conscience bothers him, by the end he is no longer fearful of shedding blood and his actions do not plague his mind. Whereas Lady Macbeth is the stronger of the two as they plan and carry out the murder of the King, by the end her mind is so destroyed by a conscience we never knew she had, that she cannot live with the knowledge of what they have done. We see a complete reversal of roles in the two characters.