Typically, the horror story genre will draw upon a reader's fear of the unknown, create a sense of dread, and include extreme acts of violence. Edgar Allan Poe's short story "The Tell-Tale Heart" meets all of these criteria.
The fear of the unknown is encapsulated by the narrator's seeming lack of a convincing motive. He professes to murdering the old man, who he supposedly "loved," and who had "never wronged" him, merely because the old man had "the eye of a vulture." The narrator says that whenever he saw the old man's eye his "blood ran cold" and that he decided to murder the old man to "rid (him)self of the eye forever." However, the narrator seems to stumble upon this motive. Trying to offer the reader a motive, after admitting that the old man had "never wronged" him, he says, "I think it was his eye! Yes, it was this!" Thus, the narrator's real motive, if indeed he ever even had one, remains unknown, and the murder is all the more horrifying because it seems motiveless.
There is also at the beginning of the story a sense of dread, as the narrator describes the painstaking preparations he took before carrying out the murder. He describes how he would, every night for a week before the murder, "oh so gently" open the old man's door and move in "slowly—very slowly." He also describes how, on the night of the murder, he opened the old man's door, "little by little," and then waited by the old man's bedside "For a whole hour." This slow, methodical build up to the description of the murder builds a sense of dread in the reader.
The horrifying, extreme act of violence is of course the murder itself and the immediate aftermath. The narrator describes how he "dragged" the old man from his bed, to the floor, and then "pulled the heavy bed over him." He then describes how he "dismembered the corpse," cutting off "the head and the arms and the legs." This gruesome act of violence is horrifying, and all the more so because of the gleeful, unrepentant tone of the narrator's description.