In the opening paragraph of the story, Montresor says:
At length I would be avenged; this was a point definitely settled...
Poe seems to have inserted the paragraph, and these words in particular, in order to establish that Montresor did not have an internal conflict. Poe's story is clearly to be about Montresor's external conflict: the problems he has in luring Fortunato off the streets and down into his doom without being recognized or ever suspected.
Montresor encounters Fortunato when his proposed victim is drunk and unaccompanied. Montresor has the problem of getting Fortunato to accompany him to his palazzo immediately without being recognized himself. Poe ingeniously helps his character solve the problem of avoiding being recognized by making Fortunato so conspicuous in his jester's costume, complete with a cap with jingling bells, that he attracts all the attention while Montresor in his black cloak and black mask is like a shadow. Many people will remember seeing Fortunato on the night he disappeared, but nobody will remember seeing anybody with him.
Montresor has concocted a story designed to get Fortunato to want to proceed voluntarily to Montresor's palazzo immediately. Montresor himself acts as if he is in a big hurry. Why? He has bought a big cask of Amontillado sherry and had it delivered to his palazzo. It is already bought, paid for, and delivered. Why is it so important that it should be judged by a connoisseur right away? This is the finely honed falsehood Montresor uses:
I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”
“How?” said he. “Amontillado, A pipe? Impossible! And in the middle of the carnival!”
“I have my doubts,” I replied; “and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain.”
“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”
Why should Fortunato want to go to Montresor's palazzo at this late hour, when he is drunk, inadequately dressed, and has a bad cold? The answer is that Montresor has told him he is on his way to see Luchesi. If Fortunato should put Montresor off, then Montresor would go straight to Luchesi, and Fortunato would have a competitor in bidding for the cargo of wine. Montresor is a poor man and could only afford to buy another one or two pipes, but Luchesi, presumably, is capable of buying an entire cargo. So Montresor's ploy succeeds. Fortunato says:
“Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.”
Thus speaking, Fortunato possessed himself of my arm; and putting on a mask of black silk and drawing a roquelaire closely about my person,I suffered him to hurry me to my palazzo.
Fortunato is wearing a "tight-fitting" costume. He is obviously not armed. But Montresor has a rapier under his cloak. Once he gets Fortunato down the stairs into his wine vaults, his victim is at his mercy. He can kill him any time he wants—but he would like to lead him all the way to the niche where the old chains are fastened to the granite wall. That would save him the trouble of dragging a body through the catacombs. Once he entices Fortunato to the spot where he can seal him behind a stone wall—for which he has the materials already prepared—his conflict is resolved and the story is nearly at an end. The whole story has been about an external conflict. Poe puts any doubts, fears, misgivings, etc., behind Montresor with the words, "At length I would be avenged; this was a point definitely settled—but the very definitiveness with which it was resolved precluded the idea of risk"—so that Poe does not have to deal with an internal conflict and an external conflict at the same time.