Stapleton, the local naturalist, is also our killer.
At the start of The Hound of the Baskervilles, detective Sherlock Holmes and his loyal friend Doctor John Watson are sitting happily in the apartment they share (as roomies) when Doctor James Mortimer arrives to tell them a bizarre family legend about a giant demon dog that haunts the Baskerville family in the southwestern English region of Dartmoor.
there is something weird going on at Baskerville Hall, and it seems to center on poor Sir Henry, the new heir. So when Sir Henry makes plans to return to his old family home, Holmes asks Watson to go with him as a kind of bodyguard/investigator combo. And in the background of all of the strange happenings that surround Sir Henry, there's always the rumor of the Hound creeping out both him and Watson.
But while Holmes is sure that Stapleton is the murderer of Sir Charles Baskerville, he has no proof that would stand up in court.
Holmes set up his final ambush of Stapleton. He and Watson talk to Laura Lyons, confirming that Stapleton demanded she set up a meeting with Sir Charles, which he then persuaded her not to attend.
Stapleton's dog is a huge animal that he's been keeping on an island in the middle of a dangerous bog called the Grimpen Mire. He's been covering the dog with phosphorus to make it glow in the dark, which is what sent Sir Charles into cardiac arrest and sent Selden running over that cliff.
Stapleton sends the dog to attack Sir Henry the night that Holmes and Watson are watching for him, but Holmes manages to shoot it before it can do Sir Henry too much damage