He shows a desire to do so, saying to himself, "That is a step/On which I must fall down or else o'erleap.Let not light see my black and deep desires." (Act I. Scene iv). Although he already holds two high titles, Macbeth still lusts for more. His newly appointed position of "Thane of Cawdor" is afterthought to his possible future kingship.
Macbeth's ambition ends up controlling him, as symbolized in the "Dagger Soliloquy," where he perceives a dagger leading him to King Duncan's chamber. This is a representation of the power his ambition has gained over him, as it is acting as a compelling force outside of his thoughts and has become something almost tangible. The real moment of his downfall comes in the third scene of the third act, after he has already committed murder against the king and taken the throne. It is important to take note of the double usage of the number three in the turning point of the play, which was at the time regarded as supernatural and was also used in the original spell in the first scene of the play. This is the moment when Fleance, Banquo's son, escapes his murder from assassins hired by Macbeth. There is a significance in this moment because Macbeth is tampering with fate, directly opposing the prophecies of the witches to attempt ensure that Fleance does not become king after he is dead.
Aside from the flaw of ambition, Macbeth also exercises selective perception, which is key to his downfall. In order to carry out that which he must do to achieve or keep the power he covets, he strategically ignores all evidence that his doings are wrong or will bear negative consequences. Instead Macbeth focuses on the information which justifies or allows for his motives. This is illustrated in the seventh scene of the first act after he has told himself all of the logical reasons that he should not kill the king. His wife, Lady Macbeth, cunningly convinces him in a speech targeted at his emotions, specifically those of pride, that in order to be a man and show his love for her, he must kill Duncan.
