Lady Macbeth is an example of pure ambition in the play. She, and not her husband, is the mastermind behind the plot to kill the king. Lady Macbeth comes up with the plan to murder the king once she discovers that he will be a guest in her home.
"It is, of course, Lady Macbeth herself who spawns the plot to kill Duncan, who determines the setting and the specific actions through which this bloody deed will take place."
She is fiercely determined to gain the throne for her husband, and would love to change places with him, if only she were the man of the family. She humiliates Macbeth into killing the king, reminding him that a real man would be willing to go through with her plan.
She makes sure that he understands what the opportunity means to both of them. She scolds him, saying that he better not complain to her if he chooses to not kill the king. She also suggests that her love for him hangs in the balance of his decision. Therefore, Lady Macbeth is the engine behind Macbeth's decision to murder the king.
Without her forceful personality, clearly, Macbeth would have chosen differently.
Lady Macbeth serves as the incentive to spur Macbeth on to more violence. She ensures that Macbeth continues with their plan, even when he starts to have doubts. Only later, when he goes on his killing spree, does she realize that she has created a monster and kills herself out of guilt.
Lady Macbeth is the one who pushes the indecisive Macbeth to commit the murder of Duncan. Earlier in Act 1, we see Macbeth waver in his thoughts about killing the king from "If chance will have me king...Without my stir," to his soliloquy in Act 2, sc. 1, where he says, when he hears his wife's bell, that this bell summons Duncan either to heaven or to hell. When he wavers in his desire to murder Duncan, Lady Macbeth attacks his manhood, telling him that if he carries out this deed, then and only then, will he be a man in her eyes. When she invokes the spirits of darkness to "unsex" her, she is the epitome of evil. Later, we see her decline into madness when by Act 5, sc. 1, she is sleepwalking, talking to imaginary people, and continually washing her hands while, always, she demands there be light around her. Her role is to show the audience how, even someone who seems to have no conscience and appears to be very strong-willed, can be brought down by evil.