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

Okay, so basically what the excerpt is talking about is that Macbeth says that usually when you kill a man, he stays dead, and doesn't come back to haunt his murderers. However, here, in this play, Macbeth constantly sees the ghosts of his victims who come back to torture his already frail psyche. These lines are important because they show how Macbeth is slowly becoming crazy, seeing things, becoming devoured by guilt.