I like the word humanity in your question--in order to answer this question you have to define what you mean by humanity. We tend to think of humanity as our kinder, gentler nature, but in the case of this story, you have to think a little more. It is hard to consider that the man who killed the old man has any humanity. Yes, he is human, but he cold bloodedly kills the old man for no good reason! And yet it is his humanity that triumphs in the end because in this story, humanity means conscience. Most human beings have a conscience that is their moral compass. For a while, the man seems to not have a conscience, but in the end, the guilt that comes from his conscious shows his humanity. After he kills the man and dismemebers the body he imagines he can hear the beating heart of the victim--those heart beats are his guilty conscience condemning his actions and his guilt overwhelms him into a confession of his sins--thus the murder is revealed and his humanity wins over his sin.