What is the main theme or Edgar Allan Poe's horror stories?
While each of his stories has some theme variation, a universal theme for all of Poe's Gothic stories is the dark nature of mankind. Poe was a Dark Romantic which means that while he agreed with the supernatural elements, focus on nature, and strict poetic structure of Romanticism, he did not share the Idealistic view of humans that many of the other Romantic authors of his day stressed.
Many of Poe's stories are narrated by unreliable, psychotic characters, but a good number of those narrators relate how they came to commit their various crimes or shocking acts, usually coupling their tale with a justification for whatever actions they took. Poe demonstrates that humans can start off seemingly normal (such as the narrator of "The Black Cat") and then allow the darkness of their souls to take hold. For other characters, he offers little reason for or vaguely hints at the motives for the characters' despicable actions, such as Montresor in "Cask of Amontialldo" or the unnamed narrator from "The Tell-Tale Heart."
What is interesting about this theme of man's dark nature is that Poe limited that darkness to males. Usually his horror stories include beautiful, vulnerable women (reminiscent of his mother and young bride) who fall prey to the maniacal male characters. It is unclear whether Poe actually viewed all human souls as inherently dark, for he certainly seems to be biased against men in this area.
Despite the different possible themes in his works, one that seems to prevail is desolation in many different ways.
It is evident in The Mask of Red Death, when Prince Prospero and his friends isolate themselves to escape an illness only to find themselves desolate, and ill, by themselves and trapped in the castle.
In House of Usher, desolation is evident in the state of mind of Usher himself, and the deterioration caused by desolation that is evident in the state of the house.
In the Raven, desolation is what is driving the young man to melancholy, as he misses his love, Lenore.
In Annabell Lee there is also the desolation of trying to reach out for his young, dead bride whose sepulchre lays in a kingdom by the sea.
Maelstrom also denotes an aspect of desolation, as the main character remembers his most bitter and saddest moments as he sinks down a maelstrom, to his imminent death (although he survives)
In all, you can see isolation and desolation combine in different shades of sadness, which are a reflection of Poe's own life story. One in which loneliness continued to permeate his psyche.
For the most part, no statement of moral value or "theme" can be drawn from Poe's horror stories. They were written for the sole purpose of creating a feeling of horror, as Poe himself expounded in his essay on unity of effect. An exception would be "The Mask of the Red Death," which deals with such themes as the inevitability of death, the superficiality of society, etc. Another story with more thematic depth is "The Fall of the House of Usher," although even there the insinuation of incest and its devastating effects upon the family could be questioned.