What shows that Lady Macbeth, after the murder, has a softer side, even if it's well hidden?

Lady Macbeth's softer side is indeed very well hidden, but it's there all the same. After Macbeth has established himself on the throne, he dispenses with his wife's advice and starts acting alone. This means that he becomes increasingly unhinged, lashing out at anyone he perceives as the remotest threat to his rule. Lady Macbeth is aware that her husband wants to wipe out Banquo and Fleance, but she isn't privy to the precise details of the plan. At this point in the play, there's not much evidence of a softer side to Lady Macbeth's personality, even if she's not actively involved in Macbeth's crimes.
Later on, however, in the famous sleepwalking scene, Lady Macbeth appears riven by guilt over Duncan's murder. With her mind rapidly disintegrating, she tries to remove the imaginary blood stains that, in her deluded state, she believes to cover her hands. Even before this, however, Lady Macbeth briefly revealed a different side to her personality: after the foul deed of Duncan's murder was committed, Lady Macbeth claimed that she felt "bold," but she was unable to bring herself to kill Duncan personally.