Immediately after he commits the murder, the narrator feels very calm and confident, he describes the whole situation in which he disarmed the body:
<em>First I cut off the head, then the arms and the legs. I was careful not to let a single drop of blood fall on the floor. I pulled up three of the boards that formed the floor, and put the pieces of the body there. Then I put the boards down again, carefully, so carefully that no human eye could see that they had been moved.</em>
Then, while he is talking to the officers, he starts feeling guilty, so guilty that he imagines the sound of the heart beating. He thinks that the officers can also hear the sound and that they are setting a trap. He ends up confessing the murder:
<em>No! They heard! I was certain of it. They knew! Now it was they who were playing a game with me. I was suffering more than I could bear, from their smiles, and from that sound. Louder, louder, louder! Suddenly I could bear it no longer. I pointed at the boards and cried, “Yes! Yes, I killed him. Pull up the boards and you shall see! I killed him. But why does his heart not stop beating?! Why does it not stop!?</em>
something she gave him probably reminded him of something bad from his childhood
