Macbeth's words reflect his internal conflict and his struggle with his conscience. He knows that murdering Duncan is a terrible crime, but he is also driven by his ambition to become king. He is trying to convince himself that the murder is necessary and that he can do it without feeling remorse. However, his guilt is already starting to consume him, and he is afraid of what he might say or do if he talks about it any further.
Lady Macbeth, on the other hand, is more ruthless and ambitious than Macbeth. She is determined to help him achieve his goal, even if it means committing murder. She tries to convince him that the murder is necessary and that he can do it without any consequences. However, Macbeth is not as easily convinced, and he knows that the murder will have a devastating impact on him.
The line "O gentle lady, 'tis not for you to hear what I can speak" is a key moment in the play, as it reveals the depth of Macbeth's inner conflict and his struggle with his conscience. It also highlights the difference between Macbeth and Lady Macbeth's characters and sets the stage for the tragic events that follow.