In Shakespeare's Macbeth, was Macbeth "sane" before he killed Duncan, or was his eventual insanity a result of the murder?
In Shakespeare's Macbeth, Macbeth seems quite sane at the start of the play. He goes into battle and fights valiantly for King and country. He is a man well respected by his peers. The idea of becoming King (after the witches' predictions) takes him by surprise. He rationally considers killing Duncan and can list all the reasons not to do it—Duncan is his cousin, his King, his friend, and his guest. Macbeth says he should be protecting the man, not killing him:
He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. (I.vi.12-16)
This is clearly a man who has a grasp of right and wrong. He is quick to realize that the only thing that pushes him to take this step is his "vaulting ambition"—a driving ambition to have more than he has (though he admits he doesn't need more) and to be King. To be more powerful means everything.
Once Macbeth crosses the line and kills his King, he knows that he is going to hell for doing it—one murder more or less will not change that. He tells himself that it will get easier with practice, and he is right. However, as he embarks on his murderous path, he begins to lose his mind—now a murderer and a tyrant, he also becomes isolated from his wife.
First Macbeth kills his best friend, Banquo—though Fleance (Banquo's son) escapes. When Banquo's ghost appears to Macbeth at the banquet, it is easy to see that Macbeth is a troubled man. His subjects begin to talk about him and his murderous behavior...about Macbeth who had so recently been praised and rewarded by his King. Lennox, a lord, speaks of all those "touched" by Macbeth's actions. Macbeth blames Malcolm and Donalbain for their father's (Duncan's) death. Then Macbeth kills the guards who allegedly murdered the King. How convenient it is that Macbeth, in rage and distress over Duncan's death, destroys the only ones who might ask questions about who really killed the King. Then Banquo is killed, but Fleance escapes. Sarcastically, Lennox says Banquo died because he was out too late at night...perhaps an excuse presented by Macbeth. However, Lennox shrewdly notes that those closest to Macbeth, who he "pities" (cares about), end up dead.
By the end of the play, Macbeth is clearly insane. He believes everything the witches tell him (though he knows they are evil) and has murdered Macduff's entire family. Over time, Macbeth loses his mind in killing so many...so very unlike the man we meet at the start of the play.