Lady macBeth is a great example of the psychological effects that murder can have on a human mind. At first Lady MacBeth is fierce, relentless, and almost male in her driving ambition. She thinks her husband is too "full of the milk of human kindness." Her greed and avarice is at its apex when she calls for the spirits which tend on mortal thoughts to "unsex her here"  and fill her up with the cruelest and direst thoughts. After the murder, which the previous poster has intimated, she begins a psychological deterioration. She still is strong, however, and it isn't until the sleep deprivation and the sleepwalking (washing) occurs that she turns into a mere shadow of herself. We see a complete negation of her wild spirit in her suicide--that is her way of escaping the guilt and her blood stained conscience.

I think that the most important moment of change for Lady Macbeth is immediately after the killing of Duncan. Up to this point her greed for the throne has blotted out every other thought or emotion she might feel. As soon as the murder is committed, she instinctively realises from Macbeth's behaviour that he will never be able to carry the scheme through in the way that she has foreseen and planned. With her ambition destroyed in an instant, she is a shadow of the woman we have seen earlier in her powerful soliloquy and her descent into despair and madness from this point seems to me entirely believable.