Answer:
As the answer above suggests, it was the main character's own guilty conscience. As we know, he is... a little mentally unstable. The heartbeat is not from the old man's corpse, it is from his own delusions and insanity from the guilt that he made for himself by murdering the old man in cold blood. Eventually, his guilt from killing the man and his fear of being caught boil over, and he confesses his crime in a dramatic breakdown.
<span>The answer would be rebuttal, because that is after one accuses the other, the "prey" (one being accused) has time for a rebuttal, to prove them wrong.</span>
