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.