It is said that ambition is the key to success. In the case of Shakespeare's Macbeth, it is the key to his downfall. He is presented with the ambition by the supernatural power of the witches. Lady Macbeth, his wife, then pushes the ambition. After the murdering of Duncan, Macbeth has gained enough ambition himself to cause his own destruction. We can see a clear building of desire throughout the play.
Macbeth is first introduced to the limits of his power and his ambitions by the witches, who greet him with three titles: Thane of Glamis, which Macbeth is fully aware of; Thane of Cawdor, which is true at this point, but which Macbeth has not been told of; and King, which has not yet become true. The witches are the ones who plant the actual idea of killing Duncan into Macbeth's mind. It must first be understood that in the Elizabethan Age, the witches would have been taken very seriously, and that witchcraft was a part of their culture. King James even wrote a book on the subject. Shakespeare foreshadows Macbeth's corruption through his meeting with these three witches. (I,iii). His thoughts are compared to Banquo's, whose morality, it seems, will not let himself turn to evil. Banquo is skeptical of the witches, and tries to warn his friend, who seems to accept what they say. Without this supernatural prophesy, the thought of killing the king would have never crossed Macbeth's mind. The thought is then reinforced when Macbeth learns that he is Thane of Cawdor, as the witches foretold (I,iii).
Now that Macbeth has the thought of becoming king inside of him, his is still not capable of killing Duncan. His morality keeps him from performing any such task. He is also fully aware of the destructive power of his ambitions. In act I, scene vii, he even tells us:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other -