Please explain Macbeth's last lines in Act III scene 4 of Shakespeare's Macbeth.
As always with such questions, it is vitally important to be aware of the context of the lines that you are trying to analyse. Macbeth's last lines come after the banquet with his lords that has been so rudely interrupted by the appearance of the ghost of Banquo that has affrighted Macbeth so and made him appear to be mad. His wife was forced to come up with some excuse to explain away Macbeth's erratic and irrational behaviour, and at the end of this scene we see them together as Macbeth determines to visit the witches again to find more answers. Note how Macbeth ends the scene:
My strange and self-abuse
Is the initiate fear, that wants hard use:
We are yet but young in deed.
To paraphrase these lines, Macbeth says that his his self-delusion of seeing the ghost of Banquo is a result of the fear of the novice that needs more experience to harden him to such qualms of conscience. The last line refers to Macbeth's acknowledgement that he is yet but "young in deed," indicating that he plans to commit further abuses of power and acts of violence and murder in the future, so that he can mature in his evil. The last lines of this scene thus express Macbeth's commitment to continue down the path of abandoning himself to evil.