Montresor's diction is quite formal in "The Cask of Amontillado," and this conveys both his intelligence and his capacity to actually achieve the thing he sets his mind to. We can tell by his vocabulary, and his use of words such as "impunity" and "retribution" and "immolation" that he has a highly developed intellect. He wants to utterly destroy his nemesis, and his diction indicates that he has the smarts to do it.
In this way, then, his diction affects the mood. Montresor is not some bumbler who we believe to be incapable of meeting his goal of annihilating his enemy; rather, his diction conveys his intelligence and helps us to ascertain that he is absolutely capable of his enemy's destruction. Thus, his diction darkens the mood, and renders it more ominous and suspenseful, because we are led to believe that Fortunato's destruction is imminent and inevitable based on what we know about Montresor.