Ambition is one thing, murdering out of fear is quite another.
It is quite correct to say that, in the beginning, after he is tempted by the witches, Macbeth thinks about killing Duncan in order to become king himself (Act 1, scene 3):
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not.
But Lady Macbeth is right: her husband is not cruel enough to kill for what he sort of wants. No, it is Lady Macbeth's "vaulting ambition" that ultimately propells Macbeth to do the deed. In fact, he knows of her desires and tells her pointedly, in Act 2, "We will proceed no further in this business." Then he gives her all kinds of very good reasons why they should not kill the king, but she prevails and manupulates him into doing the murder.
Macbeth is not ambitions, he is weak. Yes, the great and brave warrior is weak compared to his overbearingly ambitious wife. After the murder, fear overtakes Macbeth, and what looks like a wll to power is not ambition but a ferocious attemp to be safe at all costs: "For mine own good/All causes shall give way."
Macbeth's ambition is the driving force in the play. Certainly Lady Macbeth is also ambitious and thus persuades her husband to do whatever it takes to secure his position as King; however, it is Macbeth's ambition that drives the later events in the play. In the first act, Lady Macbeth says that Macbeth is not without ambition but that he lacks the drive to put his ambition into reality. So, she helps him carry out Duncan's murder. But after this is done, Macbeth is so driven to cover up his crime and continue his reign as King that he has many other people murdered including Banquo and Macduff's family. Lady Macbeth knows nothing of these murders while Macbeth is planning them, so her ambition is not a driving force for the later murders. Further, Lady Macbeth eventually repents her role in these crimes while Macbeth vows to fight to the bitter end. Thus, Macbeth's ambition is a greater driving force in the play.