In what ways has Macbeth's ambition begun to be detrimental to him?
Macbeth's ambition also blinds him to the faults in his own thinking and the long-term consequences of his actions. In these two ways, his ambition causes him to behave in a way that would have been ruinous to him even if all his plans had gone as well as possible.
Macbeth both believes and disbelieves the prophecies of the witches. If the prediction that he will become King of Scotland is true, it should be true regardless of what he does; if it is not true, it is worthless. He recognizes this,
but the driving force of his ambition will not allow him to trust in it. Had he done so, the witches' temptation would have failed, whether or not he had become king.
Macbeth's ambition leads him to the further absurdity of half-believing the witches' words. At the same time Macbeth was told he would be king, Banquo was told his descendants would be kings. Macbeth tries to defeat the second part of the prophecy with Banquo's murder, forgetting that if one part of it is untrue, the other is also in doubt.
Ambition also causes Macbeth to be consciously short-sighted. As a Christian of that era, he believes that murder and traffic with the Devil will eternally damn him. Nevertheless, he proceeds with his plans, deluded into valuing a few years as King of Scotland more than the eternity of his afterlife:
....here, upon this bank and shoal of time,--
We'd jump the life to come. (I, vii)
It is Macbeth's ambition that begins his tragic downfall. He is made Thane of Cawdor because of his bravery on the battlefield and his nobility of character. Yet, it is his ambition that immediately sets the plot in motion when he first meets the witches. He hears the witches predict that he will be King, and after a nudge from Lady Macbeth, he decides that he will have to murder Duncan to attain the title. Once he has Banquo, his best friend, murdered, it is all downhill from there. There is no stopping his reign of terror and many are destroyed including his wife and finally, Macbeth, himself. Power corrupts and ultimate power corrupts absolutely!
Macbeth becomes obssessed with the power and position of the throne. He is always thinking of people and things that may prevent him from keeping it. It's bad enough that he has killed Duncan to get where he is, but he doesn't stop there. He kills Banquo and attempts to kill Fleance; he kills Duncan's family, and he makes the people of Scotland completely miserable. It effects his judgment; his ability to sleep; his relationships with family, nobles, and his countrymen. Ultimately, his ambition leads to the death of his wife, his reign, and himself.