Never really liked this story, it's basically just Lovecraft writing Poe fanfiction. The twist is predictable even by Lovecraft standards, too.
I dont think the twist is really supposed to be surprising. This story is more a tragedy in which you're supposed to follow th struggle of the main character and feel saddened that it was all for naught and that he now sees himself as the monster that the other people see rather than a thriller or something more like what he commonly wrote.