He doesn't. Edgar Allan Poe's "The Cask of Amontillado" is set in an unnamed Italian city (often assumed to be Venice) during Carnival, but the narrator Montresor never offers a general characterization of Italians as a people. His focus is entirely on his personal vendetta against Fortunato, a single individual.
Readers sometimes project later stereotypes onto Poe's 1846 story. In reality, Poe uses the Italian setting for atmosphere (catacombs, Carnival, wine culture) not for ethnic commentary. Montresor's hatred is personal, not ethnic.