Guilt hits Macbeth and Lady Macbeth at different times and in opposite ways. Straight after killing Duncan, Macbeth is horrified by his own hands: "Will all great Neptune's ocean wash this blood / Clean from my hand?" The hyperbole shows he already feels his crime is too big to undo. He also hears a voice cry that "Macbeth does murder sleep", and sleep, a symbol of peace and innocence, is lost to them both from this point.
Lady Macbeth seems untouched. "A little water clears us of this deed" treats guilt as a stain on the skin. Yet by Act 3 she admits "Naught's had, all's spent": the crown has brought no happiness, a first sign that what they did weighs on her.
Macbeth's guilt turns into fear rather than remorse. He sees Banquo's ghost at the banquet, but instead of stopping he decides he is "in blood / Stepped in so far" that going back is as hard as going on. By Act 5 life is "full of sound and fury, / Signifying nothing", as if he has killed his own conscience. Lady Macbeth's guilt goes the other way: it is buried and then breaks out. Sleepwalking, she cries "Out, damned spot, out, I say!", and the water that was meant to clean her becomes a stain that will not go.
Context sharpens this. Many in Shakespeare's audience believed that kings were chosen by God, a view James I wrote about and promoted, so killing Duncan was a sin as well as a crime. The Old Man's "'Tis unnatural, / Even like the deed that's done" shows nature recoiling from the crime, so the guilt they feel is part of a wider judgement on the murder of a king.