How does the narrator feel immediately after he commits the murder in The Tell-Tale Heart? Do his feelings change? If so, how and why?

After planing the seemingly-perfect murder, the author executes his plan, murders the old man, and disposes of the body. We realize that he had no particular feelings of any kind toward the victim; he didn't hate the old man, nor did he like him. He is completely neutral and killed him with no particular reason.
After that, however, instead of feeling some sort of twisted, morbid fulfillment, he feels immense guilt and regret. He tries to calm down, but he fails, and he begins to hear a faint thumping sound in the distance. Confused and a bit paranoid, the murderer begins to think that the faint rhythmic sound he hears is, in fact, the old man's heartbeat. He panics and confesses to his crimes.