Blood seems omnipresent in Macbeth. While it is is one of the life-sustaining humors of the human body, it spills from the "unseamed" body of the traitor Macdonwald, and it "smokes" from Macbeth's brandished sword; it is his murderous desire and it consumes his and Lady Macbeth's imaginations, appearing and reappearing as stains upon their hands.
Short Answer: So, while blood symbolizes life, it dominates Shakespeare's imaginative drama as representative of death and murder, and it is this murderous meaning "blood on their hands" signifies. Blood also symbolizes guilt, which does not allow sleep to those who are not innocent. This is why Macbeth says "Macbeth doth murder sleep" after he has slain King Duncan.
Here are examples from Acts I and II of the use of the word blood:
- Macbeth's slaying of Macdonwald is brutal and bloody: Macbeth kills this traitor by "unseam[ing] him from the nave to th'chops" with "his brandished sword" that "smoked with bloody execution." (1.2.18,22)
Bloody means brutal and violent.
- After reading Macbeth's letter in which he relates his receiving of the title of Thane of Cawdor and the predictions of the witches, Lady Macbeth calls upon the spirits to unsex her and thicken her blood; that is, to make her hardened to compassion so that she will have no remorse about her and Macbeth's murderous intentions.
...fill me, ...top-full
Of direst cruelty! Make thick my blood,
Stop up th'access and passage to remorse (1.5.33-36-5)
- In his soliloquy of Scene 7, Macbeth muses that it would be better if Duncan's murder were the end of the affair and assure that he will be king; if that were the case he would be glad to risk his soul and life. However, by committing murder and violent crimes, those who do this teach others to commit such acts. These acts, then, come back to haunt the "teacher." Here, then, "bloody" means violent or murderous acts.
..that we but teach
Bloody instructions, which, being taught, return
to plague the inventor.... (1.7.8-10)
- Before he kills Duncan, Macbeth imagines a dagger before him. The blood on the dagger ("gouts of blood") represents the life of King Duncan, whom Macbeth plans to murder, and the "bloody business" means murder:
I see thee still
And on thy blade and dudgeon gouts of blood,
Which was not so before. Thereʼs no such thing.
It is the bloody business which informs
Thus to mine eyes. (2.1.46-50)
- After the murder of King Duncan, the Chain of Being has been broken. For the Elizabethans there was an order to the universe which must be maintained. When kings, who were directly beneath the divine in the hierarchy of these levels of creation, were killed there was a disruption in the heavens and in the world of nature. Moreover, this Chain of Being is the plot structure of Shakespeare's plays. In Act II, Scene 4, Shakespeare depicts this universal disruption as an old man talks with the nobleman Ross, telling him he recalls "dreadful and things strange" in his life, but on this "grievous night" of Duncan's death, he has seen nothing like the disturbance of the heavens ever before. Ross replies to him, and "bloody stage" means that now the Earth is out of balance with the universe as a result of the regicide committed; kings were semi-divine in the Chain of Being.
Ha, good father,
Thou seest the heavens, as troubled with man’s act,
Threatens his bloody stage. By th' clock ’tis day,
And yet dark night strangles the travelling lamp. (2.4.4-7)