Why do Macbeth and Lady Macbeth hurry to their chambers at the end of scene 2?

Macbeth and Lady Macbeth hurry back to their chamber at the end of Act 2, scene 2 because they hear someone knocking at the door. Macbeth has just murdered Duncan and Lady Macbeth has helped by smearing Duncan's blood on the drunken servants framed for the murder. As Lady Macbeth says, they have to wash the blood off their hands and look like they have been in bed, asleep, not out murdering people. She tells Macbeth to get his nightgown on and to get his act together. Macbeth is wound up at this point and recognizes that they have crossed a line that will from now on make them nervous at every knock. He says no amount of water can wash the blood from their hands, meaning that they can look innocent on the outside but the fact of their guilt remains.