This is a great question. Of course Macbeth is a study of the guilty conscience. The main characters display this more accurately than any others: Macbeth and his lovely wife, Lady Macbeth.
Macbeth first suffers from guilty twinges when he waffles with his wife about killing Duncan in the first place. He tells her that Duncan has recently honored him for his actions in battle, that Duncan is a guest in their home (therefore Macbeth should be protective of Duncan), and that he and Duncan are blood relatives. For these three reasons, he should not kill Duncan, and they will discuss it no more. However, she throws the question of Macbeth's manliness into the mix, and all is said and done.
Next we have Macbeth happily plotting Banquo's and Fleance's deaths, but he doesn't count on the ghost appearing every time Macbeth refers to his dear friend. This is an obvious display of guilt for both Duncan and Banquo's fates, and how funny it is that Lady Macbeth explains it away by telling the nobles that her husband has an "ailment" from which he has suffered since childhood. Of course, they all think about Duncan's strange death, none of them yet know of Banquo's "safety" in the ditch.
Macbeth continues to visit the witches, grow bold, and make plans without his wife. All of this shows us the distance that has grown between the two, and it is really no surprise when Macbeth plans the murders of the MacDuff family.
Macbeth's mental state, the state of his marriage, and the state of his country are all described as "sick," "wounded," "bloody." Macbeth himself tells his wife that his mind is "full of scorpions" which can only mean that he is suffering the effects of their sins.
Lady Macbeth, on the other hand, doesn't show signs of guilt until she complains that Macbeth doesn't spend as much time with her as he used to...this is the sign from her side of the fence that all is not well in paradise. From here, we have her famous sleepwalking scene, where she moans, "The Thane of Fife had a wife, where is she now?" along with her "who knew the old man had so much blood in him?" Her handwashing is an obsessive-compulsive action that she can't seem to stop since she always sees the blood and the guilt in such a palpable manner. Her nurse reports to the doctor that Lady Macbeth has been this way for many nights, and he replies that he would "not have such a heart in my bosom for all of Christendom." The doctor reports to Macbeth that what the Lady suffers from, he can not cure. He indicates that she is not physically ill, but it is something deeper. The final indication of Lady Macbeth's horrible guilt is her own suicide.
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