How do you justify that the story "The Monkey's Paw" is a horror story? You may draw references from other such stories.
The way to justify the categorization of "The Monkey's Paw" as a horror story is simply to consider what Edgar Allan Poe would call the "effect" of the story; that is, the feeling that is left with the reader after finishing it. The feeling of horror is created mainly by the ending, when someone comes to the door on a cold, dark night and begins knocking. Since White has used his second wish, saying, "I wish my son alive again," he feels sure that is his son knocking. But his son was mangled in machinery and must be a horrible sight. Mr. White knows this because he had to identify the body. His wife doesn't know what her son must look like and insists on opening the door for him. The reader can only imagine what the son must look like. It is the reader's imagination that creates the "effect" of horror and makes this a horror story.