Malcolm considers himself unfit for the throne, but seeing the corruption that has taken hold of the throne, Macduff feels that he could do no worse than that of which is directly before him.
Act 4, scene 3, page 4, line 78-106
After hearing the news of the death of his wife and all his children, Malcolm convinces Macduff to re purpose that anger to avenge the death of his family members.
Act 4, scene 3, page 10, line 227-236
Macbeth and Lady Macbeth have an argument over killing Duncan for title of King. She wins with her agrument of doing it by saying that if he came up with the idea in the first place then he should go through with it.