Edgar Allan Poe's story "Silence-A Fable" can be confusing given the story's form. The story is being told from the perspective of an unnamed listener. Essentially, it is a frame story--meaning that it exists as a story within a story.
Two characters are gathered at a tomb: the Demon and the one (the listener) the demon is telling his story to. The Demon's story is about how he tormented a man. The Demon tried turning the world into a violent one, but the man refused to flee. Seeing that this did not work, the Demon made the world silent. Unable to take the silence, the man fled.
That said, the naming of the characters as protagonists or antagonists can be rather difficult. For most, the protagonist is typically considered the "good guy" and the antagonist is considered the "bad guy." This story changes that.
One naming of the protagonist and antagonist could be defined by the two in the beginning of the story: the Demon and the listener. One could assume that the listener is the protagonist, given the story is narrated by him or her. This would make the Demon the antagonist. That said, the listener does not necessarily conflict with the Demon. Instead, he or she is only telling the Demon's story.
Then readers come across the tale. Within the tale, the man (tormented by the Demon) could be considered the protagonist--simply based on the negative connotations many readers have with Demons. This, again, would make the Demon the antagonist.
Another way to denote the antagonist and protagonist is by reversing the traditional roles. The Demon could be considered the protagonist--based upon the fact that it is his story the listener is retelling. This would make the tormented man the antagonist.
Essentially, the naming of the protagonist and antagonist is up to the reader. This is illuminated by the way the reader defines and identifies each of the characters in the text. While no answer should be wrong, be sure to support any denotation of an antagonist and protagonist with textual support.