How does not knowing what fortunato did to montresor intendify the horror of in the cask of amontillado?

Part of not knowing what Fortunato's crime against the narrator is adds to the horror because we do not know if it was something incredibly small or if Fortunato deserved it entirely, although his death was a crime in itself. Part of the terror is not knowing if the narrator is an insane, heinous murderer or a man with an agenda and a solid reason.