The essential moral issue in Macbeth seems to be conscience versus personal gain. Would you commit an immoral act if that action meant that you and your family would be financially sound for the rest of your life? I ask this question to my students, and I am often appalled by the answers. Many answer yes, even if that immoral act involved murder. But it is a moral dilemma presented often in literature: Would you sell your soul to the devil?
It is this issue that Macbeth grapples with. In his soliloquy at the end of Act 1, he lists all the moral reasons NOT to kill Duncan: Duncan is his king, he is his kinsman, he is his guest. Duncan has also been a good king. There will be public outcry against his death. There will be consequences, especially in the afterlife. Yet, Macbeth's "vaulting ambition," his desire for personal gain, outranks his moral scruples.
Lady Macbeth has no such scruples. She only sees the available opportunity of killing Duncan while he is a guest at the Macbeth castle. She knows such an opportunity will not present itself again. She does not debate the right or wrong of such an action as does Macbeth.
Yet, Shakespeare explores the effects of such an action on their psyches and on their relationship. The guilt that Macbeth feels is overwhelming and plunges his deeper into crime. "Blood will have blood." Once he crosses the line and loses his soul, he becomes a paranoid tyrant committing more and more murders. And Lady Macbeth is filled with regret.