In the "Cask Of Amontillado", why does Montresor say to Fortunato, "Yes, for the love of God"? Is he being serious? Or is he being sarcastic? Why?

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.