Lady Macbeth waits in agitation for Macbeth to do the deed. She comments that had the sleeping Duncan not looked like her father she'd have killed him herself.
Lady Macbeth isn't completely cold-blooded, foreshadowing her future feelings of guilt.
Macbeth enters. He's killed Duncan and Duncan's attendants. His hands are bloodstained and he's upset that when one of the attendants said "God bless us" in his sleep, he was unable to say "Amen." He also thought he heard a voice say "Macbeth does murder sleep" (2.2.34).
Bloodstained hands and sleeplessness: symbols of guilt. Macbeth is anguished: he knows the consequences of this murder.
Lady Macbeth soothes him and tells him to wash his hands, but notices he's still carrying the daggers he used to kill Duncan. Macbeth refuses to return to the scene of the crime. Lady Macbeth, furious, runs off to plant the daggers on the attendants.
Compare Macbeth's nervousness to Lady Macbeth's calm, collected behavior.
A knock sounds, terrifying Macbeth. He worries that not all the water in the world could wash the blood from his hands.
The knock at the door paralells the "knocking" of Macbeth's heart in scene 1.3.
Lady Macbeth returns, her hands now as bloody as Macbeth's. But she's calm, and identifies the 'mysterious' knocking as someone at the south entrance. She says: "a little water clears us of this deed" (2.2.65), and tells Macbeth to go and put his nightgown on so no one will suspect them.
Lady Macbeth is calm. She identifies the "mysterious" knocking as someone at the South entrance. But she is naïve, thinking water can wash away her guilt.
Macbeth wishes that the knocking could wake Duncan.
Macbeth shows remorse.