Lady Macbeth believes that the simple act of washing their hands will rid them of both the physical blood but also the mental guilt of their deed. She orders her husband, "Go get some water / And wash this filthy witness from your hand," (2.2), echoing this later, "A little water clears us of this deed," (2.2). Here, "water" is a symbol of purity and life. She isn't focused on what the murder says about them or the mental impact it will have, only the implications of it they get caught with blood on their hands. She worries that it will implicate them in the murder as a "witness", so tells Macbeth to "wash" it away. At the same time, she only refers to blood with the euphemisms "filthy witness" and "deed", suggesting she can't confront the reality of her actions.