Blood traditionally symbolizes violence. For the Macbeths, blood also symbolizes guilt resulting from violence. They kill Duncan in a fit of ambition and then have great regret about doing so. They often imagine blood that is not there or remember seeing blood, symbolizing their mixed feelings about their deed.
When Macbeth is trying to decide whether or not to go through with the plan to kill the king, he imagines a bloody dagger hanging in the air. Hallucination is not unusual for the Macbeths. He hasn’t even committed the act yet, but he already feels conflicted and overrun with guilt about it.
… I see thee still,
And on thy blade and dudgeon gouts of blood,
Which was not so before. There's no such thing:
It is the bloody business which informs
Thus to mine eyes. Now o'er the one halfworld
Nature seems dead, and wicked dreams abuse
The curtain'd sleep … (Act 2, Scene 1)
Blood and the visions of it follow him. Macbeth imagines the bloody ghost of Banquo at the banquet after sending murderers to kill him and his son. The sight so disturbs him that they have to send their guests home early with excuses about how it’s no big deal because Macbeth is just having a fit.
Lady Macbeth is not immune to the bloody visions. She did not kill Duncan, but she was involved in the planning and saw the body. After the murder, she took the bloody daggers from Macbeth, chiding him for not following the plan and leaving them there to complete the frame-up job. She covers for him when he sees Banquo's ghost, but she does not really know about his murder spree.
Lady Macbeth completely breaks down mentally, imagining that the blood that was on her hands that night is still there.
Out, damned spot! out, I say!--One: two: why,
then, 'tis time to do't.--Hell is murky!--Fie, my
lord, fie! a soldier, and afeard? What need we
fear who knows it, when none can call our power to
account?--Yet who would have thought the old man
to have had so much blood in him. (Act 5, Scene 1)
She eventually dies, apparently of suicide. Macbeth is shaken, and her death makes him ponder the frailty and brevity of life. It does not stop him from going to battle with Malcolm's men, a battle he is destined to lose because he is outnumbered and following contradictory supernatural advice from witches.
The witches told Macbeth that he could not be harmed by a man "of woman born," so he figured he was safe. The other prophecies seemed mostly crazy, such as the forest coming after him, but he was also told to beware Macduff.
Macbeth loses his confidence when Macduff tells him that he was not born of woman, and therefore sees him as a threat. That is the end of Macbeth. He cuts off Macbeth's head, and Malcolm becomes king.