Poe's work, like that of any great author, is not monolithic in theme or style. One could arguably make the case that within his oeuvre, contradictory worldviews are expressed—or perhaps it's more accurate that he doesn't express a fixed or defined attitude about the universe and man's place in it. His best-known works are characterized more by an absence of the sort of moral themes implied in the works of contemporaries such as Hawthorne and Emerson. Yet perhaps it's this very lack of "message" that conforms to our conception of the Dark Romantic movement or at least to some aspects of it.
Usually we define the movement as focusing upon irrational, grotesque, or gothic-horror elements within the broader tendencies of Romanticism. Like the German writers E. T. A. Hoffmann and Ludwig Tieck, Poe presents characters who are in a state of hysteria or insanity leading to violence, sadism, and murder. Usually there is no rational explanation for the behavior of these figures, and Poe's intent appears to be one of blurring the distinction between what is real and what is illusory.
In "The Black Cat," the narrator himself doesn't seem to know what has caused him to embark on a brutal spree of cruelty and killing. Here, and in other stories, there are supernatural occurrences, but these merely reinforce the possibility that the narrator is unreliable to the point of having imagined the events he describes. In "William Wilson" the double, or Doppelgänger, of the narrator who appears out of nowhere is perhaps just a projection of his mind—or even if he is real, the other Wilson represents the narrator's conscience or guilt. Poe's protagonists are crushed by forces they can't understand or control whether those forces are real or unreal.
It's facile to say that Poe's worldview is therefore that man is alone, a helpless victim of the irrational cosmos. Other stories of Poe, however, show a different picture. In "The Murders in the Rue Morgue" and "The Purloined Letter" Poe's freelance detective Dupin (the forerunner of Sherlock Holmes) solves apparently inexplicable crimes using deductive logic. Dupin is emblematic of empowered humanity, confidently using his mental powers, but paradoxically the same dark Romantic elements are present in the unreal Parisian settings and (in "The Murders in the Rue Morgue") the grotesque nature of the crimes.