Why did Lady Macbeth become evil?

Macbeth changes because of the effect of the guilt of Duncan's murder on him, making him more of an insomniac, more of a paranoid, and losing his ability to care about anyone or anything. Lady Macbeth is also changed by her part in the murder, as she finds that her husband has become a different person, her dinner parties are fiascos, and that she cannot stop thinking about all the horrible things that she and Macbeth have done.