In Shakespeare's Macbeth, although Macbeth is the tragic hero, and his tragic flaw is vaulting ambition, Lady Macbeth's ambition is just as great and is the catalyst for the destructive path that Macbeth will follow after he kills Duncan—this ambition will also destroy them.
When Macbeth returns after hearing the witches' predictions, and as he has already received the reward of the Thane of Cawdor's title and lands as they had foretold, Lady Macbeth is thrilled: if the first prediction regarding Cawdor came true, certainly it is just a matter of time (with a little nudge from the Macbeths) that Macbeth will be King...and Lady Macbeth will be Queen. This drives her as surely as Macbeth's ambition. She makes her plans for Duncan crystal clear when she and her husband discuss Duncan's departure from their castle the next day.
And when goes [Duncan] hence?
Tomorrow, as he purposes.
Shall sun that morrow see! (I.v.63-66)
However, the next time we see the husband and wife, Macbeth is having second thoughts. He likes the rewards that Duncan has been heaping on him, and wants to wait, but Lady Macbeth berates him, and shames—perhaps even frightens him—with her words. Macbeth tells his wife of his decision:
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (I.vii.34-38)
When she hears this, Lady Macbeth is furious. She insults his manhood: what led him make such an empty promise to her? Was he drunk? And if she had promised to do so, she would have killed her own infant—murdering it without a thought. She says:
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. (60-65)
I think that even as a warrior who has seen battles and taken countless lives, this is daunting to Macbeth. He decides that should she have children, they had better only be males, for there is nothing soft or nurturing in her character to say such things or to even suggest the murder of a child:
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (81-83)
This is just the beginning of Lady Macbeth's influence on Macbeth. When he finally kills Duncan—his King, his friend and his cousin...as well as his houseguest—he is so upset (for Macbeth is a man of valor not of murder), he returns to their rooms with the bloody daggers in his hands. He is extremely upset, to the point that he seems as if he is losing it. Lady Macbeth tells him to take the daggers back, but he refuses! She does so, and wipes Duncan's blood on the guards to implicate them in the murder.
Macbeth notes that over time, he will probably find it easier to kill. And it is not long before he is not only planning the murders of anyone who defies him or stands in his way, but he does it without help from Lady Macbeth.
There is no doubt that it is Lady Macbeth's ambition that drives Macbeth to become the murderous tyrant he is by the play's end. All they gain, they lose because Lady Macbeth is so ambitious. She will ultimately go insane and kill herself over what they have done, and Macbeth will be overthrown and die fighting his enemies—all for the sake of Lady Macbeth's driving ambition.