Montresor’s motive for leaving Fortunado in the catacombs is revenge for Fortunado’s insults against him.
Although he never tells us exactly what Fortunado did to him, Montresor opens the story with charging Fortunado with a “thousand injuries.” Chances are these supposed insults were minor imaginings of a raving mind, because Montresor is not quite right in the head.
Montresor is obsessed with the perfect murder. He is convinced that he is not really getting revenge unless he gets away with it. If he is caught and punished, that will not really be sweet revenge.
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.
This is why Montresor chooses to brick Fortunado up in the catacombs, where no one will find him. He will suffocate and die there, and no one will ever know that Montresor killed him.
The irony is that Fortunado has no idea that Montresor is even angry at him, so he doesn’t suspect anything. There are no signs that he is going to his death.