There has always been a great deal of discussion with regard to Lady Macbeth's heartlessness. In Shakespeare's Macbeth, when Macbeth writes to his wife that part of the witches' prophecy has come true, she cannot wait to have Duncan come under her roof (as a messenger reports his imminent arrival) so that she can do what is necessary (aid in Duncan's murder) to help her husband become King of Scotland.
Later, when Macbeth declares that he has had second thoughts about carrying out this plan, Lady Macbeth baldly states that if she had promised to do so, she would have killed her own baby:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this. (I.vii.63-65)
Throughout the remainder of the play, until the final act, this is the demeanor of Lady Macbeth: she is cruel and vicious. Duncan's blood on her hands means nothing to her, and she maligns her husband for his weakness in the face of taking the life of his King, his friend, his guest and his cousin. However, as the play nears its conclusion, the two seem to exchange places. While Macbeth becomes a master at slaughtering all who get in his way (e.g., Banquo and the Macduff family), by the play's end Lady Macbeth has started to lose her mind as her conscience torments her.
The woman who attends her tells Lady Macbeth's doctor that the Queen is afraid to sleep without a light—Lady Macbeth is afraid of the dark: this is not the woman of Act One. She now walks in her sleep, reenacting the horrible deeds she and/or her husband have committed. Because she is sleepwalking, recalling the details leading up to (and including) Duncan's murder, we can infer that her conscience is weighing heavily upon her. Over and over Lady Macbeth repeats the motion of washing her hands, to remove the King's blood—which she imagines is still on her.
Out, damned spot! Out, I say! (V.i.31)
She comments on the murder of Macduff's family—an event she had no part in, though it also haunts her. As she bemoans that the blood will never be gone—or rather, the guilt of their actions shall never be removed—she then repeats things she told Macbeth after Duncan's murder: to dress for bed to seem as if he had been asleep during Duncan's murder—to escape suspicion. She refers also to Banquo's death (another she had nothing to do with), assuring Macbeth that he has not seen Banquo's ghost, for dead men cannot come back to life.
...Banquo's buried; he
cannot come out on's grave. (57-58)
None of these things convey a mind at peace. As Lady Macbeth imagines she hears the knocking at the gate, reminiscent of Macduff's arrival to collect the King (before his murder is discovered), she tells Macbeth in her dreaming state what now sums up her current situation:
done cannot be undone. (61-62)
Lady Macbeth is later reported to have taken her own life. She has lost the ability to brush aside their ghastly actions as she did at the beginning of the play—her suicide further confirms that what they have done became too much for Lady Macbeth's conscience to bear.