A soliloquy occurs when a character speaks his thoughts aloud when the other characters are absent. Only the audience gets to hear what he is thinking.
Macbeth's seven soliloquies help us to understand his thought processes and his inner conflicts. The soliloquies are key to adding dimension to his character. If we only saw him from the outside, we might think of him as nothing more that a sociopathic killer from the start. Knowing his thoughts helps us to understand that he begins with a conscience. Through his soliloquies we track how he becomes dehumanized over time as his path of bloodshed continues.
We learn, for instance, in Macbeth's first soliloquy in act 1, scene 7, that he realizes and dreads the risks of killing Duncan. He wishes it could all end cleanly, with just this one death. However, he understands that once he starts down this road, it is likely to get bloodier and bloodier: he knows in his heart that Duncan's death won't be the end. He knows the bloodshed will come back to haunt him in this life as well as the afterlife. As he says:
Bloody instructions, which, being taught, return
To plague th' inventor
He shows himself to be human in his consideration of how wrong it is to betray Duncan. Duncan will be a guest under his roof: Macbeth, as his host, is supposed to protect him. Further, Duncan has been a good and just king who doesn't deserve to die.
We also learn that ambition drives him to contemplate this path. Ambition is his only motivation: he doesn't enjoy violence and is heartsick at the idea of killing a beloved king. He has nothing ("no spur") against Duncan personally. We learn from his own thoughts that too much ambition is his great flaw. He states:
I have no spur
To prick the sides of my intent, but only
As we move to his soliloquy in act 3, scene 1, we see some of the changes that have come over Macbeth. Gone are the qualms and worries about killing a good man. Macbeth in this soliloquy rationalizes to himself killing Banquo
. Banquo was once his good friend and does not deserve to die, but Macbeth has hardened inside and doesn't think about that. He doesn't care about his friend at all, but only about himself. He sees Banquo as a threat, because Banquo heard the witches
' prophecy and might betray him.
Macbeth is consumed with anger that he took all the risks of killing Duncan, but according to the witches' prophecy, it is Banquo's heirs who will inherit the throne. Macbeth says:
They [the witches] hailed him father to a line of kings.
Upon my head they placed a fruitless crown
And put a barren scepter in my grip.
Being king is not the be all and end all that Macbeth imagined, and now he lashes out coldly in anger against his friend. He has lost the capacity for trust and friendship. His evil deeds have made him evil. We know this because of the way he speaks his thoughts aloud, full of anger and hate.
In act five, scene five, Macbeth gives another soliloquy. By now, near the end of the play, he is devoid of fear and feeling. In this soliloquy, he reacts to the death of his wife by saying, indifferently, that she would have died anyway:
She should have died hereafter.
Then he expresses his complete disillusionment with life. He has killed a good king, along with many other innocent people, and plunged his country into war, but it is all, as far he is concerned, for nothing. His evildoing has made life meaningless for him. He says he merely goes through the motions of living day to day, and it doesn't matter to him if he lives or dies:
Tomorrow, and tomorrow, and tomorrow,
Creeps in this petty pace from day to day
To the last syllable of recorded time,
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
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,
In the end, he is nothing. His ambitions were not worth the price.
Possible theses could be something like what follows:
We learn through the soliloquies that getting what he wants through evil means has left Macbeth alone, barren, and empty, only a shell of a human being. Without the soliloquies, we wouldn't understand how dehumanized he has become.
Macbeth's soliloquies give us a window into his soul that shows that evil acts turn a person evil. Shakespeare, through the soliloquies, is showing that a person isn't born evil but becomes evil through the decisions he makes.