Is the narrator wrong in thinking Fortunato is out to get him or is Fortunato two faced?
The narrator, Montresor, doesn't think Fortunato is out to get him. He says that Fortunato had insulted him, and that is why he decided to get revenge. That's all we get from Montresor regarding the insult. He never tells us more about how Fortunato had insulted him. Because he doesn't give any details, this makes many readers wonder whether Fortunato ever did anything to Montresor; perhaps it is all in Montresor's mind. Most people who are driven to murder have a specific reason why they want to kill someone.
Another thing that makes the reader feel that Fortunato's insult might never have happened is that Montresor tries to convince us that he is isn't crazy. Why does he feel he needs to even bring this up? He keeps repeating throughout the story that he is sane.
When Fortunato realizes what Montresor is doing to him, Fortunato doesn't seem to have any idea of why Montresor has chained him to the wall and is leaving him to die in Montresor's family catacombs. If there had ever been any trouble between the two men, I think Fortunato would have mentioned it while begging for his life. He seems truly shocked.
It is never clear what Fortunado has done to incur such wrath from Montressor. The only evidence against him is that he is something of a wine snob and a bore, but other than that, nothing. The story begins:
"The thousand injuries of Fortunato I had borne as I best could ; but when he ventured upon insult, I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged ; this was a point definitively settled - but the very definitiveness with which it was resolved, precluded the idea of risk. I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
It must be understood, that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation
It is, however, possible that Fortunato is trying to play the fool. One common defense mechanism of most people is to act dumb when confronted with a serious issue. It's possible that Fortunato thought he could convince Montressor that no ill will was ever intended.
The only flaw to this line of thinking is that Fortunato never does admit to any deed. Most times when someone is acting dumb and it doesn't work, they will excessively apologize for their actions in an attempt to save themselves. Fortunato never does this...