William Shakespeare's Macbeth is a tragic play. A tragic play possesses a tragic hero (Macbeth). This tragic hero possesses a tragic flaw (hamartia). In the case of Macbeth, his tragic flaw is ambition.
The plot of the play illustrates what one will go through in order to better himself or herself. In order to better himself, Macbeth must murder the king. That said, he never would have even considered murdering the king if he had not met the witches. After murdering the king, Macbeth must hold onto his title. In order to do this, he must continue to take lives which he believes could take the throne from him. His ambitious nature pushed him to murder Duncan. His growing ambition forced him to continue his murdering.
The characters in Macbeth also further the theme of ambition. The witches put the idea of being king in Macbeth's head. Lady Macbeth challenges Macbeth's manhood. Macbeth murders Duncan, Banquo, and Macduff's family in order to feed his ambition.
The language of Macbeth also furthers the theme of ambition. When Macbeth does decide to take the throne, he states that he does not want anyone to know what he is after ("Stars, hide your fires; / Let not light see my black and deep desires"). When finding out about the prophecies, Lady Macbeth questions if Macbeth "has it in him" to do what must be done. She believes that he lacks the ambitious nature necessary ("Art not without ambition, but without / The illness should attend it").