It's simple in that he's out for revenge, he knows what he's going to do, and he pursues it objectively. The complexity is not of enormous size, but of a small confusion as to why he did this and that. The reader may have reason to wonder why there was compassion, when the two were descending into the catacombs and Fortunato's health was of concern to Montresor. There may also be curiosity as to why Montresor showed the emotions he felt when he sealed Fortunato's fate. Poe does a great job of turning great writing into great storytelling, which causes readers to process and reflect on certain topics