I went down to open it with a light heart, — for what had I now to fear?
The narrator here describes his mood as he goes to open the door for the police. From this statement alone, the narrator appears to feel no guilt, but it is perhaps more accurate to say that there is no outward manifestation of his guilt. He thinks he has nothing to fear because he has physically hidden away all evidence of the murder. However, he does have something to fear: himself. Sitting alone with his guilt in the spot where he committed the crime proves emotionally too much for him to handle.
But anything was better than this agony! Anything was more tolerable than this derision!
This quotation comes just before the end of the story, as the narrator describes himself listening in horror to what he believes is the old man’s heartbeat as the police enjoy his torment. In reality, the police hear nothing and suspect nothing until the narrator confesses his guilt. The narrator’s guilt is so powerful that it feels like an external force tortuously acting upon him. Because he knows that he’s guilty, he believes the police must know he’s guilty.