Rather than sarcasm, it is a more sophisticated form of verbal irony. Although the two are closely related, all verbal irony is not sarcasm. Here, Montresor is responding to Fortunato's pleas for help, but not in the way Fortunato wants. Instead, Montresor is telling him of all the pain, punishment, and inequality of the world in one line, and essentially saying, "Yes, this is why you're here-for the love of God." This is the moment when Fortunato (and perhaps the reader, if he/she has not already) realizes how mad Montresor truly is, despite all his claims to sanity.
Montresor's conversations with Fortunato are marked by his penchant for twisting Fortunato's words for his own use. For example, shortly before this, Fortunato says it is time to be getting back to the palazzo, where his wife and others are waiting. He says "Let us be gone," and Montresor repeats, "Yes…let us be gone." These are his subtle jests with his victim, his revelations that the world is not as Fortunato imagines. Indeed, Montresor is not as Fortunato imagines, and the knowledge leads to his death.
He is being somewhat sarcastic by echoing Fortunato's own words. Fortunato asks to be set free "for the love of God," and by replying with that same phrase, Montresor almost implies that what he is doing (killing Fortunado) is for the love of God. One can imply this because the last paragraph seems to indicate that Montresor is not happy about killing Fortunado, in fact he is "sickened" by it, but completes the act any way by placing the last brick. However, Montresor, especially compared with Fortunato, is a very serious character. I would say the only sarcasm is in using the same words; he is actually serious about doing what he does "for the love of God." Although he is sickened by the act, he does not indicate that he feels any guilt.