The correct answer is - The news that lady Macbeth has killed herself.
Even though Lady Macbeth made her husband kill the king, it seemed that she was plagued by guilt more than the indecisive Macbeth. He grew accustomed to murder in order to achieve his goals, whereas his wife slowly started descending into madness. Ultimately, she killed herself, and her husband wasn't even surprised - he became desensitized to death.