Lady MacBeth quotes

I.
"What's done cannot be undone." (5.1.71)
"What's done is done." (3.2.14)

When Lady Macbeth says "What's done is done." in the third act after Duncan's death it is obvious how she seems to feel towards the ex-king's death. She wants to make MacBeth realize that there is no longer any need for fretting because he had already committed the act. She also is making herself realize that there is no need to worry, because this is after she sees Duncan's body and becomes anxious.

When she goes back to each death in her mind and on her transformation of MacBeth in the sleep walking scene in act V she is obviously feeling quite different. When she says "What's done cannot be undone." she is having doubts that what they did was proper. Also she knows that MacBeth truely is a killer because he has killed more than just Duncan (a man who in her mind deserved it.) She says that it cannot be undone in order to point out that there is regret for taking part it what has occured, this is in comparision to her noncommital line of what's done is done.