I don't think Fortunado deserved his fate. I have no idea what he did to insult Montresor, but I am sure it was not as bad as all that. Montresor is conceited and definitely focused on revenge, but that is not the same thing as justice.
Clearly, justice is a key theme in this chilling short story. Whether we believe him or not (and certainly many, including myself, consider him an unreliable narrator) Montresor certainly takes justice into his own hands. Note that it is not enough merely to bring about the downfall of Fortunato or to pay an assassin to bump him off - no, for Montresor, ever-eager to fulfill his family's motto, he must be the one to engineer and design a foul ending for his enemy that he must administer by himself. To Montresor it is as if justice (from his perspective) can only be administered personally. And administer it he does.
In this story, obviously, the protagonist has taken justice into his own hands to administer as he sees fit, which is, in this case, bricking up his antagonist to die! This raises lots of ethical and moral questions, obviously, about justice and how it is administered and how it should be administered, etc. For example, in many countries, the police forces are not useless and only present to pretend to have authority in communities...they are only there for show. Citizens are left to fend for themselves and they are freely allowed to simply administer their own justice against robbers, murderers, etc. A good example of this is in Guatemala. A group of men robbed some friends of mine that live there (they are native Guatemalans and they own a store in Palin); they have a police force, but they do not come investigate, etc. They are present merely for show, so it is an understood and accepted practice for citizens to track these people down themselves and either hurt or even kill them! This is shocking, yes, and very sad, in my opinion, but they truly believe in the concept of "an eye for an eye" there. This is only one example, as well, but shows how people in different countries, etc., view justice.