In "The Cask of Amontillado," there is no reason to believe that Montresor committed murder before he killed Fortunato. The reason his says "the piles of bones of which I have before spoken" is because he described them earlier in the story.
There is tons of imagery in this story, and the description of the catacombs aids in creating the eerie mood. Describing the pile of bones that lays by Fortunato's final resting place is an example of such imagery. Also, Montresor placed those bones there to cover the mortar and tools he uses to entomb his "friend," which suggests that this act was premeditated.
No. Montresor, the narrator, is describing the crime that he committed fifty years ago. When he makes reference to the piles of bones, he is referring to the catacomb where he has brought Fortunato to murder him. The catacomb is lined with mounds of human skeletons and bones.