"The Cask of Amontillado" is told in the first person. How does this help to heighten the effect of horror that Poe wishes to produce?

By writing in the first person, Poe allows the reader to be inside Montressor's thoughts and witness firsthand the inner mind of a murderer.  Probably the most significant element is the reason Montressor gives for killing Fortunato-- an "insult".  Because Montressor does not elaborate, the reader never does find out the nature of this "insult"; however, given the fact that Fortunato willingly goes to the catacombs with Montressor, it couldn't have been very significant in Fortunato's mind.  If the insult was as petty as "I don't like your taste in wine", it becomes all the more horrific that Montressor would resort to murder for something like that.  If one insult could set him on the path to murder, it might leave the reader wondering if he ever repeated the crime for some other "insult".