I'm not so sure that the story was meant to be a horror story, primarily. Certainly a horrible thing happens in the story, the mangling to death of poor young Howard, but the story is just as philosophical as it is a tale of horror.
After all, much of the horror is in the mind of the reader, for no conclusion can be drawn as to why Howard died. And did Howard's disfigured, bloody corpse claw its way out of the grave, trudge all the way to his parents' home and knock on there door in the dark of night because of a wish? Or was it but the wind and your imagination?
Chance or fate? Who's to say? And how can one hope to prove one's theory one way or the other?
A wish was made. A death occurred. Money was obtained. There was a late-night knocking at the door. So?