In a way, the narrator was betrayed by his own heart if we consider his conscience his heart. He killed the old man without any thought of the consequences, because he had an evil eye. He even admitted that the old man had done nothing against him. Yet he killed him.
At first, the narrator was so convinced he would get away with murder that he let the police in, and even invited them to sit right above the old man’s dismembered body, which he had hidden under the floorboards. Yet, soon something started to go wrong. He could swear he heard a heart beating.
It grew louder—louder—louder! And still the men chatted pleasantly, and smiled. Was it possible they heard not? Almighty God!—no, no! They heard!—they suspected!—they knew!—they were making a mockery of my horror!-this I thought, and this I think.
The narrator’s conscience, or heart, is making him hallucinate the beating of the old man’s heart. The old man is dead. His heart is not beating. The narrator does not really hear it. The police do not hear it. Yet, in his head it is there. It convinces him to betray himself, and give himself up, and tell the police where the old man’s body is.