In Macbeth, we do not see Lady Macbeth for two Acts. What do you think has happened in the interim?How can you explain the changes in her character?
Excellent question! You have identified one of the biggest changes or transformations in this masterful tragedy that shows some of the consequences of unbridled ambition and acts of violence. When we see Lady Macbeth again - I assume you are referring to Act V - it is clear that she is tortured by what she has done and her inlvolvment in the murder of King Duncan. She confesses her complicity unknowingly to an audience, with the incredibly powerful symbol of hand washing. It is clear that her mind is overcome by feelings of guilt and sorrow for her actions, which in the end are responsible for her death. In a sense, having given over her body to the forces of evil to gain for her husband the kingship she wanted him to have, she has given herself over to madness and is not able to shake from her being the knowledge of what she and her husband have done, no matter how hard she tries - consider the plea in her famous "Out damn'd spot!" speech. As a side issue you may wish to consider if we feel pity for Lady Macbeth by the end of the play, or whether we feel she got exactly what she deserved.
My own humble opinion is that she was the third murderer, to aid her husband as she continually did throughout the play, and in her meddling she saw them kill Banquo, she aided them in killing Banquo, and therefore she'd seen the blood herself. Just as her husband before her, the actual act of holding a knife in her hands and being an essential part of the deed being done shook her. She mentions earlier that she didn't perform the murder of Duncan herself because he appeared like her father- this seemed a little too much like an excuse to me. So, if this is how events actually unfolded, then it may just be that the killing of Banquo had the same affect on her as the murder of Duncan had on Macbeth - at least immediately. That could have been the reason that ultimately led to her demise, but of course this is just my opinion.