In Shakespeare's Macbeth, what does this quote by Macbeth mean? It will have blood: they say, blood will have blood. (III.iv.123)

In Shakespeare's Macbeth, my
understanding of this quote has always been that "blood begets blood," or that if blood
is shed, it means more bloodshed will follow.

In Act III,
scene four (a great scene for those who like the supernatural), Banquo's ghost has
visited Macbeth several times. The Elizabethans believed that if a person had met with a
violent end, he might walk the earth. Macbeth is having a banquet and keeps saying he
wishes Banquo were there, and so Banquo shows up, but no one else can see
him.

Macbeth's
comment...

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"...the time has been. / That when the brains
were out, the man would die."
(III.iv.79-80)

This is almost
a comical line: it used to be that when you killed a man, he stayed dead. His remarks,
in that he alone sees Banquo, sound
ridiculous.

Macbeth has been a little unbalanced since he
murdered Duncan—of which Lady Macbeth reminds him—so his reaction to ghosts,
most especially something he cannot fight, pushes him to his wits'
ends. He admits:

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What man dare, I dare: / Approach like the
rugged Russian bear, / The armed rhinoceros, or the'Hyrcan [wild] Tiger, / Take any
shape but that, and my firm nerves / Shall never tremble...
(III.iv.101-105)

I can fight
the most frightening of animals that any man could face (bears, tigers, the rhinoceros),
but I cannot face ghosts.

Macbeth's
comment "blood will have blood," is followed by a list of unnatural occurrences that
began to take place the night he murdered Duncan. The Elizabethans also believed it a
mortal sin to kill a king because God ordained (decided) who would be king. With
Duncan's death, there is a disruption to the universe: the wrong man sits on the throne.
And until that changes, earthquakes and eclipses will occur; and
gravestones will move and trees will speak.

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Stones have been known to move and trees to
speak. (III.iv.125)

So
Macbeth believes there is more bloodshed to follow: bloodshed breeds bloodshed. However,
his closing comment for the act promises that he will be personally
responsible for some of this bloodshed:

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...My strange and self-abuse / Is the initiate
fear that wants hard use; / We are yet but young in deed.
(III.iv.14-16)

In other
words, the visions he has seen are simply due to a lack of experience with murder: with
more experience, he'll feel better; and, after all, they've only
just begun.