How is the sense of horror heightened at the beginning of Act 1 scene 2 of Macbeth
As admirers of Alfred Hitchcock will argue, violence that occurs off stage or screen can be more powerful than violence that occurs on. At the beginning of Act 2.2 (I assume you're referring to Act 2.2, though that is an assumption, since you don't identify the act), we learn that Macbeth has, indeed, assassinated Duncan. Though the murder takes place off stage, it carries force and creates horror. And the bloody daggers make the assassination concrete; they provide physical evidence.
The horror is increased, too, by suspense. Macbeth hears voices and Lady Macbeth hears "the owl scream and the crickets cry." Duncan's sons, it's revealed, are sleeping in the chamber next to where Macbeth killed Duncan and were still awake (they were just finishing a prayer).
Perhaps ironically, the suspense the audience feels is partly, during this scene of the play, created by our worries that Macbeth and Lady Macbeth will be caught. We can't help it. The play centers on Macbeth and Lady Macbeth throughout most of act one and Act 2.1. That creates the same effect point of view does in fiction: it makes the reader/audience identify with them. We don't want them to get caught at this point. We will later, but at this point we are in part on their side. The threat of exposure creates suspense and adds to the horror.
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