In Act I and II, Lady Macbeth is a driving force behind Macbeth's actions. She is bold and ruthless, and when she reads the letter from Macbeth describing the witches' prophecy, she swears that she will push Macbeth, who she fears is too soft, to do whatever is necessary. To this end, she asks to be "unsex'd," so that she might be bold and strong.
When Macbeth wavers before committing the crime, she points out that he had vowed to do what was necessary to fulfill his destiny, and says that she would commit horrible acts before she would forget a vow:
I have given suck, and know
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.
Essentially, she challenges his manhood, and encourages him to let go of any remorse he might have. It is obvious that she loves her husband, but her ambition for him and for herself is overwhelming. She is the strongest character in the first half of the play.
By Act V, Lady Macbeth no longer plays an important role in Macbeth's life, and she is suffering from what her physicians says is an "infected mind.". She is torn apart by guilt, as the famous "Out, damn'd spot!" scene demonstrates:
Here's the smell of the blood still. All the perfumes(45)
of Arabia will not sweeten this little hand. Oh, oh,
By the time she commits suicide near the end of the play, she is a shell of her former self.