How does Macbeth discuss what it means to be human and have a conscience?
Macbeth is fully aware of his conscience. Guilt and fear occur to him almost immediately after he considers killing the king. He must really work up to killing Duncan. His wife encourages him but in the end, he must psyche himself into a mental state to overcome his fear and future guilt. At the end of Act I, Scene 4, Macbeth has not yet killed Duncan. But he is already trying to hide his murderous thoughts:
Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see. (I.iv.57-60)
He doesn't want the stars shed light on his evil thoughts. He is afraid for the "eye" to see what he is thinking and what he might do. One could interpret that Macbeth is afraid of what God or other people think of him. But this idea of hiding his guilt has to do his own conscience. It's as if he doesn't want his own "eye" (mind or conscience) to see or acknowledge the evil that he is considering. So, to be human, is to wrestle with one's own urges and ethics. This is a Freudian analysis in the sense that Macbeth is wrestling with his own superego and Id: his respective reason and "deep desires." Macbeth feels at the mercy of a war between his conscience and his desires.
Because Macbeth never masters his urges and emotions, he blames fate and circumstance. He feels more like a puppet than a person. He is never able to use his free will and conscience to override his urges. Therefore, he concludes that his free will had been an illusion. Here is part of his famous nihilistic quote in Act V, Scene 5:
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. (V.v.26-30)
Macbeth feels like an actor. His life had been scripted and therefore his own will has meant nothing. Having never mastered his own reason and desires, in the end he feels like life is simply a matter or being manipulated.