Early in the story, the author mentions "except the murder in Chapter Six."
I was unable to find any direct reference to a killing or murder in Chapter Six ("The Banishment"). This leads me to suspect that either there was an error on the part of the author, the "murder" is a type of metaphor, or there is something in the words or wording itself.
I have several ideas on this, but don't want to say too much yet.
the narrator is referring to a book he was reading, not to this book he is currently writing. see the opening sentence to chapter 2 "Ten years ago I read a novel about a clandestine social club..." It's chapter 6 of this novel he means. Not sure which book he is referring to or if it contains any clues? Best on your treasure hunt.