The most important words spoken by Sherlock Holmes are when he reveals who the killer is.
The main character is Sherlock Holmes, a private detective. Holmes is brilliant, but he does not like to let people in on his thought processes. He usually does not tell anyone, even Watson, what he is thinking—until the end. Watson notes of Holmes “that he was exceedingly loth to communicate his full plans to any other person until the instant of their fulfillment” (ch 14, p. 103). So when Holmes reveals the murderer, it is quite an event. Until then, no one knows who it is except Holmes himself—and the murderer!
Holmes tells Watson: “I am giving you some information now, in return for all that you have given me. The lady who has passed here as Miss Stapleton is in reality his wife.” (ch 12, p. 87)
This is the point when the reader can come to realize that Stapleton has a secret. He has in fact come to the moor under an alias, pretending his wife is his sister, so that he could get close to Baskerville and kill him.
Holmes is convinced that he will catch Stapleton red-handed.
“It is murder, Watson—refined, cold-blooded, deliberate murder. Do not ask me for particulars. My nets are closing upon him, even as his are upon Sir Henry, and with your help he is already almost at my mercy." (p. 88)
In fact, Holmes is correct. Stapleton did kill Charles Baskerville, and tried to kill Henry Baskerville. Holmes prevented it.