It is a foreboding, or ominous, setup, and clearly signals the evil and dread that will exist throughout the play.
Right from the start, we get a feeling that nothing good will happen in this play.
Macbeth will kill King Duncan, using the guards' daggers.
He will then smear the King's blood on the passed out guards, framing them for the murder.
King Duncan's son, Malcolm, reports to his father that just before he was executed, Cawdor confessed, repented, or apologized, and asked for forgiveness.
King Duncan reacts sadly, and talks about how Cawdor had been a trustworthy friend.He then tells Malcolm that one can never tell what is in someone's heart by looking at their face.This foreshadows, or signals, what is to going to happen between King Duncan and Macbeth, who Duncan knows as a fiercely loyal soldier and subject.After Macbeth comes home, he tells Lady Macbeth that King Duncan will be over for dinner that evening.Lady Macbeth tells Macbeth that he must use this opportunity to kill King Duncan and take over the throne.Lady Macbeth tells her husband that he must be the perfect host so he won't arouse any suspicions, but also be ready to act when the time comes.Macbeth, however, is having second thoughts about killing King Duncan.Macbeth killed the guards to keep them from protecting the King and serving as murder witnesses, and to blame them for his own crime.But he claimed to have done it out of raging, uncontrolled anger over their killing his King.There are several important scenes in Act 1 of Macbeth.First there is the exposition on the very first page, which sets the dark and evil tone for the rest of the play.