Blood is an important image in Macbeth. It appears throughout the play. A few examples follow. First, when Lady Macbeth wants to steel herself to the murder of Duncan, she asks the spirit world to "make thick my blood," equating "blood" with courage. Second, after Macbeth murders Duncan, his king, blood becomes the symbol in his mind of his great guilt. He asks whether all "Neptune's ocean" can wash the blood (guilt) from his hands and decides that no, instead, the blood on his hands would turn the green oceans red. Lady Macbeth accuses him of cowardice for such words as she dips her hands in Duncan's blood and says "My hands are of your color [ie red with blood], but I shame/to wear a heart so white [cowardly]." Again, she equates blood with courage.
For Lady Macbeth, however, blood changes over the course of the play from a symbol of courage to a symbol of guilt. The blood she so proudly wore now haunts her. She begins to sleepwalk--as Macbeth said earlier, Duncan's murder had "killed" sleep--and as she sleepwalks, she tries to wash the blood from her hands:
Out damned spot! ...who would have thought the old man [Duncan] to have had so much blood in him ...
As she continues to sleepwalk, she also begins to smell the blood that haunts her, saying that all "the perfumes of Arabia" would not remove the scent.
These few examples begin to show how powerfully blood represents the murder of the innocent in this play and to reveal the guilt that accompanies murder.