What effect did ambition have on Macbeth and Lady Macbeth?

Arguably, Macbeth is a play about what happens when people allow ambition to run wild and completely take over. In contrast to Banquo, who prays to heaven to restrain his ambition in act 2 when he says, "Merciful powers, / Restrain in me the cursèd thoughts that nature / Gives way to in repose" (2.1.9–11), or the humble Malcolm, who inherits the throne after Macbeth's death, Macbeth and Lady Macbeth both allow their ambition to completely control them and are duly punished.
In the opening of the play, Macbeth and Banquo hear a prophecy about their future. Macbeth is told that he will be both Thane of Cawdor and the new king, and Banquo is told that his descendants will be kings. Macbeth, upon hearing the witches' prophecy, is at first as disbelieving and hesitant as Banquo. However, once he has proof that part of the prophecy is true, he becomes ambitious. Instead of struggling against his ambition like Banquo does, he prays that "Stars, hide your fires; / Let not light see my black and deep desires" (1.4.7–8). Macbeth welcomes a prophecy that allows him to be king, while Banquo fears it and does not dare hope that his descendants might one day sit on the throne.
Macbeth communicates the prophecy to Lady Macbeth. When Lady Macbeth hears this, she exclaims,

Come, you spirits That tend on mortal thoughts, unsex me here, And fill me from the crown to the toe top-full Of direst cruelty. Make thick my blood. Stop up th’ access and passage to remorse, That no compunctious visitings of nature Shake my fell purpose, nor keep peace between Th’ effect and it (1.5.46–53).

Lady Macbeth encourages Macbeth to kill the king and his attendants so that Macbeth can take over the throne. However, this ambition soon spirals out of control. Macbeth commits more and more murders to keep his throne secure until there is an rebellion against him and his tyranny. Lady Macbeth is wracked by guilt, which drives her to insanity and ultimately to her death. Both of these characters are consumed by their ambition until it destroys them.
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