Why are these lines from act iii important in the plot of the play?

macbeth to lady macbeth

the time has been,
that, when the brains were out, the man would die,
and there an end; but now they rise again,
with twenty mortal murders on their crowns,
and push us from our stools: this is more strange
than such a murder is

These lines are important because they give us an insight into Macbeth's slowly declining mind. He is becoming crazier by the day, devoured by his ambition and guilt. The excerpt above tells us that Macbeth is becoming angry at seeing the ghost all the time because he thought once you kill a man, he remains dead. However, he keeps seeing his ghost as a symbol of his guilt and growing madness.

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