This story of Poe's is more of an adventure/mystery tale than a horror tale like many of his other stories. The mood is also just that; the reader feels like there is a good mystery to be solved, and is along for the ride, engaged in enjoying the details of the mystery as they are revealed.
Going along with the theme above, there is no clear-cut antagonist; the antagonist is possibly not a character, but an assumption. The narrator assumes throughout almost the entire thing that Legrand is mad. It is this assumption that provides a lot of the conflict and contention in the story. The entire time the reader is left questioning Legrand's sanity instead of more thoroughly enjoying the treasure hunt that he takes us on. The freed slave Jupiter is too helpful to be the antagonist, Legrand is not the antagonist, and the pirate Kidd, although a potentially good candidate for antagonist, has no live, active role in the story. It is the constant questioning and bewilderment of the narrator that provides the contrast and conflict; hence, it is the antagonist.