Complete Monster: Mr. Abney from "Lost Hearts" (link) is a seemingly avuncular and kindly caretaker who takes in his orphaned 11-year-old cousin Stephen. Stephen soon notices the ghosts of a little Gypsy girl and a young Italian boy prowling the grounds, and it is revealed Abney himself is an occultist who wishes to achieve great power. To this end, Abney takes in children and once they hit a certain age, he cuts out and consumes their hearts which also condemns them to fate as lonely specters. Stephen is his next intended victim, and Abney uses his own journals to gloat over how ridiculous the concept that human justice can apply to him is, seeing nothing wrong with sacrificing children for his own benefit.
Nightmare Fuel: Made more effective by James' leaving a lot of room for imagination.
Interestingly, he did not use this tactic in "Canon Alberic's Scrap-Book," whose climax is laid out in almost loving detail. "Pale, dusky skin, covering nothing but bones and tendons of appalling strength; coarse black hairs, longer than ever grew on a human hand; nails rising from the ends of the fingers and curving sharply down and forward, gray, hony and wrinkled. [Dennistoun] flew out of his chair with deadly, inconceivable terror clutching at his heart. The shape, whose left hand rested on the table, was rising to a standing posture behind his seat, its right hand crooked above his scalp. There was black and tattered drapery about it; the coarse hair covered it as in the drawing. The lower jaw was thin—what can I call it?—shallow, like a beast's; teeth showed behind the black lips; there was no nose; the eyes, of a fiery yellow, against which the pupils showed black and intense, and the exulting hate and thirst to destroy life which shone there, were the most horrifying feature in the whole vision. There was intelligence of a kind in them—intelligence beyond that of a beast, below that of a man." Not a story to be read alone, in an isolated farmhouse, with the sky dimming in the west and the wind howling down off the prairie. Hypothetically.