When Macbeth kills Duncan earlier in the play, Lady Macbeth has to go back and return the daggers to the dead guards so it looks like they were the ones who killed Duncan. When they hear the knocking at the gate she says "a little water clears us of this dead", referring to the blood on both of their hands. At this point in the play she is very casual about the murder and still led by her ambition. In 5.1, this idea of blood being on her hands has completely consumed her and even though it is not apparent her subconscious still sees it and it's impossible for her to get her hands clean enough.
Darkness is an image that is used often in the play as well. In 5.1, the reader learns that Lady Macbeth asks to have a candle with her at all times. This shows that she has become afraid of the darkness that earlier she so easily welcomed. Also, it is implied that even though her eyes are open she is asleep and cannot see--another type of darkness.
When Macbeth kills Duncan he says that he hears voices calling out that "Macbeth has murdered sleep"--sleep is nourishing and important, and by killing Duncan Macbeth thinks that he has ruined everyone's ability to sleep soundly (mostly his own). We see these images return in Lady Macbeth in 5.1 because she is sleep-walking. So, in a way, Macbeth was right--he 'murdered' her ability to sleep soundly because of the actions they both took.
The only way this scene redeems Lady Macbeth is that it shows she does have a conscience. For so much of the play she is so strong, ambitious, and ruthless--she has no issue with shaming Macbeth into killing Duncan to get what she wants. As Macbeth grows in his own ambition and blood-lust, we do not see as much of Lady Macbeth, but it helps here to know that she actually does feel bad about all of the murder and it's catching up with her subconscious and killing her.
