There is some evidence of mental disorder or illness in Macbeth. We have characters hallucinating and acting irrationally.
Macbeth clearly has some kind of mental disorder. First he imagines a floating dagger and believes that it is leading him to kill Duncan. Then, he imagines that he hears a voice calling him a murder.
Me thought I heard a voice cry “Sleep no more!
Macbeth doth Murder sleep” (Act 2, Scene 2, p. 30)
Macbeth is frightened by the fact that the voice says “amen” and he cannot say it back. After he kills Duncan, things go from bad to worse. Macbeth hallucinates again, but this time there is an audience. After he has Banquo murdered, he imagines he sees his ghost at the banquet. He thinks the ghost is accusing him of murder.
Thou canst not say I did it: never shake
Thy gory locks at me. (Act 3, Scene 4, p. 40)
If people were not suspicious of him before, they will be now!
Lady Macbeth is not immune to mental disorder. She seems quite disturbed when she discusses bashing her child’s brains out to prove her ruthlessness. However, her husband’s murderous streak and her part in it is too much for her. The guilt drives an already unstable mind mad. The gentlewoman tells the doctor that Lady Macbeth has been sleepwalking.
Since his Majesty went into the field, I have
seen her rise from her bed, throw her nightgown upon her,
unlock her closet, take forth paper, fold it, write upon't, read(5)
it, afterwards seal it, and again return to bed; yet all this
while in a most fast sleep. (Act 5, Scene 1, p. 76)
Lady Macbeth is driven mad by her guilt, and she sees Duncan’s blood on her hands. She cannot sleep, and she spends all of her time sleepwalking and trying to wash the blood from her hands. Then, she kills herself.