Start with a thesis statement. A good thesis statement is not a statement of fact. It needs to be your opinion about the story. It also needs to be an opinion that is debatable AND defensible.
A good way to start that kind of thesis statement is with the word "although." That subordinating conjunction allows you the opportunity to admit to any counter arguments that your reader may have, and then provide evidence as to why your opinion is the correct one. For example: "Although Montressor's hurt pride allows him to feel entirely justified in his murder of Fortunado, he still goes to great lengths to hide the crime."
That thesis statement allows you discuss Montressor's character as well as the events leading up to the murder. It also gives you a wide opening to point out all of the little details that Montressor had lined up in order to get away with the murder in the first place.