How did Macbeth evolve as a different individual in the last few scenes of the play? Did circumstances change him.. or was it his guilt coupled with over confidence/assumptions made by himself?
I do think Macbeth changes somewhat in Act 5, but he does not change completely. He is still violent, self-centered, and impulsive. Macbeth certainly becomes very poetic when his wife dies. He gives one of the most beautiful speeches in the play about life being worthless, "a tale told by an idiot" that means nothing.
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.(30) (Act 5, scene 5, p. 84)
He is ready for battle, but then when he finds out that Macduff was in fact not "born of woman" he gets nervous and basically gives up. He believes the prophecy still, so he has not changed that much. However, he loses his courage and bravado and basically becomes a whimpering coward.
Macbeth morphs into the person he is based on the ambition that is planted in his mind by the witches in Act I. He allows his love for his wife and desire to please her to spur him into action and murder the King. Once this is done, his soul is gone--he has murdered sleep and his castle is now the "gateway to Hell" as depicted by the Porter. Macbeth must secure his new title, and what better way than to continue murdering those who may rise up against him? Banquo and Fleance are first on the list, and then Macduff and his family pose a problem. Of course, Macbeth is spurred on by the illusions presented to him by the witches and the apparitions giving him confidence that he should not have. They purposely mislead him because they enjoy toying with mortals. His "tragic flaw"--ambition--is planted, watered and harvested from beginning to end. Malcolm's army marching up the hill disguised as trees give Macbeth a hint of his downfall, but he banks on the prophecy that "no one born of woman" can harm him. It is only when Macduff reveals that he was born by C-section that Macbeth realizes he is in trouble.