The curse made Baskerville vulnerable, because he was afraid of the hound since he thought it was the cursed Hell Hound.
Sir Charles was essentially scared to death. When he saw the dog, he thought it was the supernatural hound. He felt that it was after him, and his family’s curse was his doom. In reality, Stapleton just took advantage of the legend to get Sir Charles off balance and kill him.
Their evidence, corroborated by that of several friends, tends to show that Sir Charles's health has for some time been impaired, and points especially to some affection of the heart… (ch 2, p. 12)
Since Sir Charles was vulnerable, it was easy to take advantage of him. His weak heart and some creativity tricked him into thinking an ordinary dog was the supernatural curse. Stapleton’s plan would have worked in Holmes had not realized what was going on.