At the beginning of the play, Lady Macbeth is the driving force in that she is the one who persuades Macbeth to murder King Duncan. Lady Macbeth says that her husband is "too full o' the milk of human kindness" to kill King Duncan, and indeed, in act one, scene seven, he tells his wife, "We will proceed no further in this business."
Lady Macbeth then sets about undermining Macbeth's masculinity. She asks Macbeth, "Art thou afeard?" She also asks him if he is prepared to "live a coward." Macbeth initially replies that he "dare do all that may become a man" but that "Who dares do more is none." However, Lady Macbeth persists and tells him to "screw (his) courage to the sticking-place." Macbeth eventually gives in and agrees to murder the king.
At the end of the play, Lady Macbeth is reduced to a sleepwalking shadow of her former self. She imagines a "damned spot" of blood on her hands that she can not wash out. She declares that even "all the perfumes of Arabia" can not hide the smell of the blood.
This imaginary spot of blood symbolizes the guilt that Lady Macbeth can not remove from her conscience. This guilt is the cause of her downfall. At the beginning of the play, she is decisive, determined, and bloodthirsty. By the end of the play, she is guilt-ridden, sleepless, and pitiful.
It is also tragically ironic that Lady Macbeth, at the end of the play, can not remove the imaginary spot of blood from her hands. At the beginning of the play, after the murder of King Duncan, she tells Macbeth that "A little water clears us of this deed."