Here's why:
* He's trying to convince himself and the reader: The narrator is deeply aware of his guilt and the horrifying act he committed. He believes his actions were driven by a "vulture eye" that he perceived as evil.
* He's masking his guilt: He tries to present himself as rational and controlled, but his increasingly erratic behavior reveals the truth. His obsession with the sound of the beating heart, which only he can hear, highlights his heightened anxiety and paranoia.
* He's trying to avoid responsibility: By insisting on his sanity, he tries to distance himself from the consequences of his actions. He's projecting his own guilt onto an external force, the "evil eye," thus avoiding acknowledging his own agency in the crime.
The narrator's insistence on his sanity is ultimately futile. His attempts to convince himself and the reader are increasingly hollow, ultimately leading to his confession and downfall.