Macbeth is a broken man by Act 5.  His arrogance, paranoia and instability lead to his downfall.

In the beginning of the play, Macbeth is a hero. His battlefield praises are sung, and King Duncan gives him a promotion.  But Macbeth has been visited by three witches, and as a result he is only interested in more power.  His wife convinces him to kill the king and take the throne.  All goes well, but as with many who have power, Macbeth soon begins to fear he will lose it.

To be thus is nothing,

But to be safely thus. (Act 3, Scene 1, p. 42)

Next Macbeth kills his friend Banquo, and the family of Macduff.  These moves are designed to cement his lineage on the throne, but Macbeth gets more and more nervous.  He asks the witches to tell him more.  This time, with Hecate now, they give him several warnings.  Macbeth finds all of the warnings implausible and ridiculous, so he figures he is safe.

At the beginning of Act 5, things start to really go downhill for Macbeth.  Lady Macbeth kills herself after saying some incriminating information that the doctor and servants are pretending they did not hear.  His own scouts are telling him that armies are coming for him.  He begins to doubt his safety.

Bring me no more reports; let them fly all!
Till Birnam Wood remove to Dunsinane
I cannot taint with fear. What's the boy Malcolm?
Was he not born of woman? (Act 5, Scene 3, p. 80)

Macbeth is beginning to be aware that he has been cementing his own downfall.  He decides that he will go out fighting.  When he does come into contact with Macduff, his behavior is ridiculous.  He assumes he is safe from Macduff because he was "born of woman."  Yet when Macduff tells him he was born by C-Section, Macbeth seems to lose all faith in himself, and basically gives up.  He keeps fighting, but the next time we see him Macduff comes onstage with his head.

In the end, Macbeth was the victim not only of his own ambition but of others.  His wife and the witches, including Hecate, pushed him to the limit.  He was like a kid trying to hit a pinata, turned so many times he did not know which way was which.  He is manipulated by everyone, but in the end not able to hold on to his position or his power.