Macbeth is presented by Shakespeare early in the play as a brave and honoured soldier. Why is this important?
This is important for several reasons. First, the sergeant's description of Macbeth's deeds in battle delight Duncan, giving the audience a chance to see how highly he thinks of Macbeth. Duncan exclaims, "O valiant cousin! Worthy gentleman!" when he hears how Macbeth has killed the rebel Macdonwald. But the gory details also show that Macbeth is capable of extreme violence. He essentially split the man open with his sword, cut his head off, and put it on the top of the castle. The first point underscores the treachery Macbeth engages in by murdering a man who put so much faith in him; the second foreshadows some of his violent deeds later in the play. More generally, the presentation of Macbeth as a noble man at the beginning of the play allow him to explore the many ways in which ambition, and, arguably, the supernatural, can destroy people's lives.
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