If I understand your question, Macbeth's soliloquies help develop pathos for a character who otherwise we would simply view as a villain. From the beginning we are allowed inside Macbeth's mind as he contemplates the image of the slaughtered Duncan and shudders. Later we see him agonizing over the ethical reasons that Duncan should not be killed: Duncan is a good king, he is Macbeth's kinsman, he is Macbeth's guest. Even after he commits the murder of Duncan, Macbeth's soliloquies show us that this murder has only made Macbeth paranoid, fearful that his ill-gotten gains may be taken away, and so the unhappy Macbeth plans a further murder. Throughout the play, Macbeth's soliloquies show the audience that the murders have not brought Macbeth any degree of happiness, security, well being. Instead, they have caused him to lose his wife, his honor, his friends, the respect of his countrymen. In one of his most eloquent soliloquies, Macbeth's reflection shows his despair:
My way of life
Is fall'n in to the sere, the yellow leaf;
And that which should accompany old age,
As honour, love, obedience, troops of friends,
I must not look to have (Act 5, scene 3)
This keen awareness of the consequences of his actions, the misery that has resulted, make us understand the workings of a guilty conscience. While we do not condone Macbeth's actions in any way, we do to some extent understand his pain and this understanding makes the audience somewhat more sympathetic than we would be without the soliloquies. Don't get me wrong, Macbeth's actions are horrific, and he should feel pain and remorse. But the fact is that many murderers do not feel the agony that Macbeth feels throughout the play. In this way, we can identify much more easily with Macbeth who succumbs to temptation, digs himself deeper, and suffers greatly than we can to other villains, such as Iago in Othello, who feels no such guilty pangs.