Macbeth, the brutal warrior of Act I who is referred to a "Bellona's [the goddess of war]bridegroom" defeats Macdonaldwald and is awarded the title of Thane of Cawdor by the king for his bravery. Yet, knowing her husband, Lady Macbeth fears his nature:
It is too full o' th'milk of human kindness/To catch the nearest way. Thou wouldst be great,/Art not without ambition, but without/The illness should attend it. (I,v,16-18)
So, in order for Macbeth to attain what Lady Macbeth can except for his trepidation, she continues,
And that which rather thou dost fear to doThan wishest should be undone. Hie thee hither,/That I may pour my spirits in thine ear,/And chastise with the valor of my tongue/All that impedes thee from the golden round/Which fate and metaphsical aid doth seem/To have thee crowned withal. (I,v,24-29)
Thus, Lady Macbeth becomes the driving force for Macbeth's ambition to be king. She "unsexes" herself and goads him into killing Duncan when he visits their castle.
However, as the play progresses, it is Macbeth who surpresses conscience to desire, killing recklessly, while Lady Macbeth finds her conscience and, guilt-ridden, commits suicide.
At the beginning of the play, Macbeth is a good man, a nobleman, and a respected warrior. Lady Macbeth, on the other hand, is ruthless and ambitious. When Macbeth reveals the witches' prophesies to his wife, it is Lady Macbeth who wants to take things into their own hands to make them come true. She is excited that it has been foretold that her husband will be King, and wants it to happen right away. She plots to kill Duncan, the present King, so that Macbeth will ascend to the throne immediately, and is afraid only that Macbeth is too soft, "too full o' the milk of human kindness" (I,v,17) to take the direct route to fulfill their ambitions. She scorns her husband for not having the ruthlessness to kill Duncan so his own quick accession to the throne will be assured.
Macbeth is essentially goaded by his wife into killing Duncan, but once he has embarked on the road of immorality and murder, he continues with a vengeance, even though he is at times wracked by guilt. Lady Macbeth, on the other hand, is tormented by remorse to the point that her sanity is threatened and eventually lost. As Macbeth arranges to have Duncan, Banquo, Fleance, Duncan's attendants, and Macduff's family murdered, Lady Macbeth descends into madness, sleepwalking in the night and envisioning blood on her hands. While Macbeth, driven now by ambition, arises to forcefully secures the throne, Lady Macbeth loses touch with reality and sinks to the point to where she finally kills herself.
At the beginning of the play MacBeth is a soldier who listens to the advice and suggestions of Lady MacBeth, his figurative "general." At first MacBeth is indecisive and cannot see the larger battefield that is their lives. He meets with the weird sisters and is baffled by their prophecies. It is Lady MacBeth who urges MacBeth to "screw his courage to the sticking place" and literally make the prophecies come true. When her husband leaves the murdered Duncan, she returns the daggers to the scene of the crime and smears the guards with blood. Lady MacBeth is at her bloodiest and most cruel. She has prayed to the dark powers to take her milk for gall and fill her up with the "direst cruelty."
Later in the play, (after the psychological festering of their guilt) each of the characters become what the other is not. MacBeth becomes hollow and without feeling and Lady MacBeth becomes disturbed, weak, and ends up killing herself. How does MacBeth react to this? He offers one of the most nihilistic speeches in Shakespeare--his "tomorrow and tomorrow and tommorrow" speech. MacBeth has changed from the passionate soldier into an unfeeling empty shell of a man.