The purpose of any soliloquy is to reveal more about the thoughts, feelings, personality, mindset and motivations of the character(s). In case of Macbeth, The soliloquies reveal the moral and emotional turmoil going on inside his heart and soul. Macbeth would have been less of a tragic hero and more of a bloody villain, had we not witnessed the scorpions in his mind and suicidal despair revealed in the soliloquies.
Macbeth’s soliloquies before the murder of Duncan show the vigorous internal struggle of himself, as his conscience is fighting against his evil minds. In Act I, scene iii, when Macbeth is thinking about the "two truths" told by the witches, the idea of murder appears for the first time "My thought, whose murder yet is but fantastical, shakes my single state of man"(I, iii, 139- 140); however, at this point, he vehemently rejects the idea of murder, "If chance will have me king, why, chance may crown me, without my stir."(I, iii, 143-144) The predictions by the witches may have strengthened the criminal intentions that he had probably never yet dared to express clearly, even to himself; yet he is not alliance with crime; but obviously temptation is working upon him.
As Duncan makes the announcement of Malcolm as heir to the throne, Macbeth starts wondering if murder is the only way in which he can achieve the kingship. His ambition overcomes his finer nature. He calls upon the stars to hide their light, indicating that his "black" desires come out, and he thinks it is too evil to be seen. Macbeth’s image of the eyes’ winking upon the work of the hand is expressive both of his intense aversion to the deed and of his intense desire to get what the deed will accomplish. At the same time his "let that be" marks the point at which his fascinated contemplation of the thought of murdering Duncan becomes a resolution, although he will waver from it. In Act I, scene IV, shortly after Duncan’s arrival to Macbeth’s castle, Macbeth gives voice to his feeling concerning the rashness and the awfulness of the projected murder:
If it were done when ‘tis done, then ‘twere well it were done quickly… But here, upon this bank and shoal of time, we’d jump the life to come. But in these cases we still have judgment here. (I, vii, 1--8)
At this point, Macbeth’s character has fallen to the point where he has the desperate courage to commit the murder, but is afraid of the consequences. His three reasons against the murder are over-ruled by Lady Macbeth, and Macbeth determines to proceed to the murder. From that time, he plunges into a life of evil.
Still, it’s important to stress the imaginative tensions in Macbeth’s character. In the next soliloquy just before the murder of Duncan, Macbeth sees the fearful vision of a blood stained dagger leading to him to Duncan’s chamber. He addresses the hallucination of the dagger. He tries to grasp it but cannot and knows it is the product of his overheated brain.
Just after killing Duncan Macbeth continues to murder his way in the frantic desire for peace of mind en-route evils. The great bond that links him to other human beings does virtually disappear, so that the pursuit of his desire for inner peace makes him careless for anything life has to offer. Macbeth spinning his dehumanization utters the most poignant soliloquy - “… My way of life/ Is fall’n into the sere,/…As honour, love, obedience, troops of friends,/ I must not look to have,…” but in their stead/ Curses, … mouth honour, …” (5.3.23-29)
Thus at the news of his wife’s death, he responds in low key and bitter. In one of the greatest speeches in all of Shakespeare, he accepts the news with a horrifying calm:
“She should have dies hereafter.
Life’s but a walking shadow, a poor player
That struts and freets 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.” (5.5.16-27)
This famous speech acknowledges fully the empty mockery his life has become. His life has become an insane farce, not because he no longer has any power or physical security, but because he has ceased to care about anything, even about his life.
The theatrical metaphor quoted in the last soliloquy resonates throughout play. Macbeth has, in a sense, tried to seize control of the script of his life, to write it in accordance with his desires, in the clear knowledge. Thus all of the soliloquies of Macbeth become a close scrutiny of study of evil and of a conflicting soul of Macbeth’s personality.