It is ironic indeed that Lady Macbeth, of all people, should find herself wracked with guilt over the murder of Duncan. After all, she was the moving spirit behind this foul, treacherous deed, the woman who constantly cajoled and incited her husband to commit this ultimate act of treachery.
Not only that, but she even called upon the spirits that tend on mortal thoughts to harden her heart, make her more of a man and less of a woman in order to help her go through with the dastardly deed.
And yet now, with her husband sitting on the Scottish throne, something she wanted more than anything else in the world, she's suffering pangs of consciousness over the leading role she played in Duncan's murder.
Lady Macbeth's guilt manifests itself in the terrifying hallucinations she has when wandering around at night in Dunsinane Castle. As she sleepwalks, Lady Macbeth imagines that her hands are covered with blood. She tries to clean her hands, but the blood won't come off. Not even all of “great Neptune's ocean,” in the words of Macbeth, will be enough to wash all the blood from her hands.
This is because Lady Macbeth's soul is steeped in guilt. There's nothing she can do now to atone for her role in such a wicked act. It's too late to turn the clock back now; what's done is done, as she herself said to Macbeth.
As there is no way to change what happened, there is no way to expunge the grief from the soul. Somehow, Lady Macbeth will just have to live with it. That she's unable to do so, that her grief eventually drives her to madness and suicide, indicates just how powerful it is.