Lady Macbeth and Macbeth have three soliloquies between them before the murder of Duncan. The dramatic purpose of these is twofold: first, it lets the audience know the main characters' thoughts in the pivotal and suspenseful time before the murder of a monarch. Second, the soliloquies build up a mood and atmosphere of chilly and relentless terror, depicting a world taken over by the dark side.
In act I, scene v, Lady Macbeth receives the letter from her husband telling her the witches' prophecies. She is as ambitious as Macbeth is and wants the crown as much as he does but fears he will be too virtuous to do what it takes to realize their joint lust for power. Therefore, she calls on dark spirits while talking herself into a state of pitilessness and hardness so that she can goad her husband into performing the worst of deeds, tyrannicide. In lines such as the following, she build up the mood of foreboding and horror that will continue to grow, asking for a heart:
Of direst cruelty. Make thick my blood.
Stop up the access and passage to remorse
Meanwhile, in an important soliloquy
in Act I scene vii, Macbeth begins to realize the enormity of what they have planned. He realizes that once they commit the murder there is no going back. He understands, too, that Duncan's death will not be the end, but the beginning of a path of endless bloodshed, saying:
Bloody instructions, which, being taught, return
To plague th' inventor
He begins to think, too, of what a good ruler Duncan has been and how personally generous he has been to Macbeth. Macbeth is talking himself out of an evil deed, one he describes in terms that chill us as an audience.
However, as Lady Macbeth's soliloquy warned, she manages to persuade her husband to act despite his better instincts. In his soliloquy in act II, scene i, on the eve murdering Duncan, Macbeth imagines seeing a bloody dagger and wonders if it is real, then pictures creatures of the night unleashed, such as wolves and witches, and nature upended in supernatural ways, bringing the Gothic horror of the opening scenes of the play to a crescendo with the declaration that this night Duncan will either go to heaven or to hell.
In probably the most famous soliloquy in the play, in Act V, scene v, Macbeth's wife has committed suicide and Macbeth describes a life that is a contrast to what he had hoped to achieve with the murder of Duncan. The soliloquy completes the arc that began with the murder plot: Macbeth has gotten what he wanted, the crown, and yet, it has left him empty and miserable, wishing for nothing more than to get through a dreary existence, stating:
Tomorrow, and tomorrow, and tomorrow,
Creeps in this petty pace from day to day
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,
This soliloquy makes the moral of the play clear: murdering one's way to power leaves one feeling that life is meaningless and sterile.
Thus, the soliloquies outline the moral (or immoral) trajectory of the play and lay bare its moral lesson.