As of the end of Chapter 5, there is only one real likely motive for the killing of Sir Charles Baskerville. That motive is money. We know from Chapter 5 that various people stood to get important inheritances from Sir Charles. Among them are the butler, Barrymore, Mortimer, who was Sir Charles's friend, and Sir Henry. One might also suspect the Desmonds, who inherit if Sir Henry Baskerville is killed.
At this point, none of these suspects has been cleared. Holmes has sent a telegram to try to determine if Barrymore could be the person stalking Mortimer in London. But no reply has yet been received.
In Ch. 5 after a pleasant lunch at the Northumberland hotel in London Sherlock Holmes, Watson, Sir Henry Baskerville and Dr. Mortimer retire to a private sitting-room to discuss their plan of action. In the course of their conversation we learn that the real motive for Sir Charles Baskerville's murder was not merely money but also the ownership of the vast Baskerville estate which was estimated to be about a million pounds. It is this information - the high price of the estate - which causes Holmes to remark that a desperate man will be ready to risk his life for such a large amount of money:
The residue all went to Sir Henry.”
“And how much was the residue?”
“Seven hundred and forty thousand pounds.”
Holmes raised his eyebrows in surprise. “I had no idea that so gigantic a sum was involved,” said he.
“Sir Charles had the reputation of being rich, but we did not know how very rich he was until we came to examine his securities. The total value of the estate was close on to a million.”
“Dear me! It is a stake for which a man might well play a desperate game.
Several beneficiaries are named in Sir Charles Baskerville's will and it was Sir Henry Baskerville who benefited the most - a sum of seven hundred and forty thousand pounds. But this pales in comparison to the actual value of the physical assets which Sir Henry will inherit - the estate worth about a million pounds!!
In Ch. 5 Holmes strongly suspects a "man with a black,full beard," and when Dr. Mortimer tells him that Barrymore, Sir Charles' butler is a "man with a black, full beard," Holmes suspects Barrymore. However, a telegram sent to Baskerville Hall confirms that Barrymore has been there and so his name is cleared.
After Barrymore's name has been cleared Holmes does not suspect anyone in particular.