In Macbeth, how does Macbeth's character evolve over the course of the play and how is he affected by his involvement in Duncan's murder?
Macbeth evolves from a hero to a murderer throughout the course of the play.
In the beginning of the play, Macbeth is a hero. He is a soldier who acts bravely on the battlefield. His praises are sung before King Duncan, who honors him with a promotion to Thane of Cawdor.
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave… (Act 1, Scene 2)
When Macbeth informs Lady Macbeth that the witches have prophesized his kingship, she is thrilled. She worries that he will be too soft to do what is needed, so she carefully plans out a murder plot and tells him what to do step by step.
It is after he murders Duncan that Macbeth begins to really change. He did not want to be a killer, and was nervous before and directly after. However, it seems to do something to him. He goes from not wanting to kill to a veritable killing spree.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’ (Act 3, Scene 1)
Macbeth decides that to be safe he has to kill Banquo, even though his friend has supported him at every turn. Even that is not enough, and despite Lady Macbeth’s objections that he is not telling her things, he kills Macduff’s entire family. This last action proves to be his downfall, as the fiery Macduff vows his revenge, and ultimately gets it.
Macbeth's character development follows a definite arc based on the theme that ambition is destructive. When the story starts, he is almost timid. As he gets momentum, he seems to not be able to stop. But his destructive behavior only accomplishes his goals temporalily, as it also brings about his doom.
Macbeth's character deteriorates over the course of the play. At the very start we have a view of him as being a noble and valiant warrior, an image instantly undone by his murdering Duncan. On the surface he appears increasingly morally corrupt as he resorts time and again to murder, but as we get an insight into his thoughts throughout the play, we realise that this is the result of his mental disintegration, after killing Duncan. He is unable to cope with the guilt over this first murder (as is Lady Macbeth) and it drives him to act ever more recklessly. After killing Duncan, all his other acts essentially arise from desperation at the course he has set. This leads him ultimately to an utterly bleak view of life, that it has no value, no meaning at all. However, at the very end of the play, when facing Macduff in mortal combat, he does regain a modicum of his former bravery as a fighter, refusing to give in even although everything is lost.