In Shakespeare's "Macbeth", we see a lot of evil; we see evil people and we see evil acts. We can see that - almost - everyone has an evil desire within them. However, only a really evil person acts on that impulse and commits something morally wrong. An evil deed doesn't necessarily mean an evil soul; Lady Macbeth was not naturally evil. Before her and her husband came to the conclusion of killing, they had their doubts. How can someone be pure evil, if they doubt the evil that they want to commit?
Lady Macbeth's immediate thoughts (after receiving the letter from her husband) may make her appear as an utterly nonreligious, cold and ambitious woman, but this is not so. To prepare for what she feels must be done she calls on evil spirits to, "stop up th' access and passage to remorse", and if she had not done this her conscience would not have allowed her to act relentlessly.
This shows us that despite what we see of Lady Macbeth she is very unsure of herself and her thoughts. When she sais to her husband, "look like the innocent flower, yet be the serpent under't", she was being hypocritical because we see her as being the "serpent" - to her husband and when she's on her own -, yet "underneath" she is just confused and unsure of herself. Someone so confused and unsure of them selves surely couldn't be pure evil.
We can see that Lady Macbeth knows and loves her husband. Lady Macbeth is sure that being king is what her husband really wants and that killing is the option best for both of them. She knows him so well that she believes that he may be, "too full o'the milk of human kindness, to catch...