In Macbeth, a number of things appear to contribute to the title character's downfall. The witches's prophecies and his wife's influence certainly spur him in his actions. It is something more internal, though, that is truly at fault. Macbeth's ambition is one factor that causes him to commit the crimes he commits, but it is Macbeth's impotence and his desire to prove himself a man that is really at fault for his nefarious deeds.
In the first act of Macbeth, Macbeth is coming home from a victorious battle and is met by three witches. They call him Thane of Cawdor and future king. They also predict that his friend Banquo will be the father of many kings. When Macbeth returns to court, he learns that he is now Thane of Cawdor, which was one of the witches's prophecies. He then begins to to think that to fulfill the prophecies he may have to kill Duncan, the current king of Scotland, but he decides to leave it up to fate. He goes to his castle and tells his wife the good news and of the prophecy that he will become king. She thinks he should murder Duncan as soon as possible, but Macbeth is wary. Lady Macbeth procedes to completely immasculate him, calling him a coward and a woman. For a man who is impotent and already may feel insecure with his manhood, being called a woman by his wife caused him to kill Duncan, for the most part, to prove that he is really a man.
Macbeth's need to prove his manhood is not limited to showing up his wife. He is insanely jealous of men who are able to rear children. The witches predict that Banquo will be the father of many kings. Even though Macbeth is king, he cannot enjoy it because he knows he cannot produce an heir, so one of Banquo's line will take over the throne. Macbeth kills Banquo and attempts to kill his son out of pure jealousy. For this same reason, he murders Macduff's wife and family. Macbeth cannot come to terms with his impotence, so he makes his enemies suffer by murdering the signs of their virility, their children.
Even Fate seems to mock Macbeth's inability to produce children. The visions the witches show Macbeth consist of a bloody child and a crowned child holding a tree. The bloody child represents Macduff, who was ripped from his mother's womb and is the only man who can defeat Macbeth. The child holding a tree symbolizes Malcolm and Macduff's army that hides behind tree boughs to attack Macbeth's castle. Even Malcolm's army is made up of "unroughed youths," young men who have not grown beards yet. Macbeth is defeated by an army of children. It is truly ironic that Macbeth's desire for children is what defeated him in the end.
Macbeth is essentially a play mocking the male
mentality. All of Macbeth's problems, all of the crimes he commits are fueled by his persistant need to prove his manhood. His impotence, not ambition or fate, is what causes his downfall.