The real murderer was Beryl Stapleton.
According to Pierre Bayard, Beryl wanted to kill her husband for his affair with Laura Lyons (then her primary motive became marriage to Sir Henry). To this purpose, she decided to frame the completely accidental death of Sir Charles as a murder and persuaded Dr. Mortimer to ask Holmes to investigate. She then shadowed them in London in disguise as a hansom cab passenger and stole Sir Henry's shoe. When Watson arrived to investigate, she successfully created the image of her husband as a jealous and dangerous man. Selden's death, while also an accident, played neatly into her hands. Finally, on a crucial night she let the dog loose and told her husband that the hound was gone - while indeed removing or even altering the trailmarks. After Stapledon drowned, she planted Sir Henry's shoe on the islet, tied herself up and waited for arrival of our sleuths and completed the set-up by lying to Sherlock about the Stapletons' backstory, with Holmes none the wiser (BTW, Bayard doesn't buy that Stapleton even was a Baskerville to begin with).To top that up, Bayard surmises that Beryl was actually possessed by the spirit of the girl who had died 150 years because of Sir Hugo Baskerville. Thus the entire story was actually a revenge against the Baskervilles - as it should logically follow that, some time after her marriage to Sir Henry, Beryl is going to conjure an "accident" which will leave her as a sole heiress to Baskervilles' vast fortunes.
Oh, and the hound was never a particularly aggressive type. It was coated by phosphorus just because Stapleton was afraid that it would get lost in the mire.
