Macbeth and Lady Macbeth are both ambitious, but Lady Macbeth has more drive and her husband has more paranoia.
Lady Macbeth is convinced that her husband just does not have the ability to get things done. He is too nice!
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene 5)
She is right about that. Macbeth does have ambition. When the witches tell him that he is going to be king, he pretends to laugh it off but really just becomes obsessed. When he learns that Duncan did not name him successor, he gets really annoyed and his ambition rises to the surface.
Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see. (Act 1, Scene 4)
Lady Macbeth is also right about the fact that Macbeth does not have the “illness” to attend to ambition. He is not crazy enough to try to get things done even if he wants them to get done. As a result, he manages to talk himself out of killing Duncan until the vision of a bloody dagger talks him in.
It is mainly because his wife concocted such a careful plan and made sure he followed it to the letter that Macbeth was able to kill Duncan. He did not stop there though. It turns out that Lady Macbeth had created a monster. She did not know what to do, because once he started killing he did not seem to stop. Macbeth's bloodlust, paranoia, and momentum kept him killing until Macduff finally took him out.