Montresor is unbelievably thoughtful and careful when executing this murder in revenge for the wrongs Fortunato has done him. He knows just how to manipulate people to avoid suspicion, and his thoughtful planning at each stage of the murder is evidence that he commits the perfect crime. First, he claims that he never "gave utterance to a threat"; he tells no one about his plans to kill Fortunato. Further, he says,
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will.
He allows Fortunato to continue to think that they are on good terms so that Fortunato does not guard himself against Montresor's machinations.
Next, Montresor has chosen a night in which he can be in costume: a night during the carnival season. After he finds Fortunato in the streets, drunk, Montresor dons a "mask of black silk" and draws a long, black cloak around him so that no one will be able to identify him. Anyone who sees the two men together will not know that it is Montresor underneath the black costume, and so he will not be connected with Fortunato's disappearance.
Third, Montresor emptied his house of servants without appearing to realize that he has done so. Rather than give everyone the night off, which could raise suspicions later, he gives them "explicit orders" that they should "not [...] stir from the house" though he, himself, will not be home all night. Of this, he says,
These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.
Finally, because of the manner in which Montresor disposes of Fortunato, there is literally no evidence of the crime. There is no blood, no body to hide or get rid of, and no murder weapon to be found. By walling Fortunato up in such an extensive underground vault, Montresor all but insures that no one will ever find Fortunato's body.