Alternate Character Interpretation: Enough that Stanley Sargent wrote The Black Brat of Dunwich which recast Wilbur as the hero, seeking the Necronomicon in order to banish his more monstrous brother back to big daddy Yog-Sothoth.
Anti-Climax Boss: Wilbur, the hybrid son of an Eldritch Abomination, is unceremoniusly killed by a guard dog of all things. A single guard dog, not even a pack, and judging for the description of the scene, the guy (who is even revealed then to be grotesquely inhuman) barely put a fight at all.
Fridge Logic: Where did Wilbur's Y chromosome come from? Males inherit that from their fathers (females have XX).
If Wilbur is anything like Helen Vaughan, then he might not actually be biologically male—he could have assumed a shape agreeable to most people's image of physical maleness, just as Helen Vaughan wears a pretense of physical femaleness but is, in fact, an eldritch monstrosity that changes sex according to the situation.
Narm: Describing a fight with an invisible monster is hard enough to do seriously in third person narration. It becomes even worse when the reader's viewpoint doesn't actually make it to the climax of the story and is instead forced to settle for listening to a description of the fight from a guy watching it through a telescope, making the ending come across like listening to someone describe a bad mime routine to a blind person.
The Woobie: Lavinia. She's unattractive, deformed, isolated, lives in squalor with a batshit insane father, and is eventually killed by her unholy spawn whom she had begun to fear. To make matters worse, it seems that her father' and son's souls escape the demonic whippoorwhills at their deaths, but she, they managed to catch. Even after death, she can't catch a break.