Fortunato isn’t a classic protagonist. He’s a pretty creepy guy and he’s not so bright. He doesn’t exhibit any heroic qualities and pretty much walks right in to his own underground casket. He seems driven purely by addiction, with a little pride thrown in for good measure. He gets our sympathy because he is tortured and murdered, not because we get to know him or care about him personally.
Montresor is a pretty unlikely protagonist unless we think of Fortunato as addiction personified. If Fortunato represents Montresor’s addiction, something which has injured and insulted Montresor and which Montresor finally conquers by killing it, then Montresor becomes a hero.