Intended to incarnate ill will and malice, Stapleton is conflated at various points with the lecherous libertine Hugo, whom he resembles. Stapleton is a black-hearted, violent villain hidden beneath a benign, bookish surface.
If Hugo operates as a kind of Doppelganger for his entomologist heir, then the convict offers an interesting parallel as well. Serving mainly as a red herring in the mysterious death of Sir Charles Baskerville, the convict also operates as a foil for the real culprit, Stapleton. Personifying "peculiar ferocity," "wonton brutality," and even dubious sanity, the convict is shown to be a pathetic, animalistic figure on whom the detectives ultimately take pity. Not so with Stapleton, a man with a "murderous heart," and a wolf in sheep's clothing.
Stapleton is a worthy adversary because of his birthright. If the convict is a simple murderer, he is also simply born, related by blood to the Baskerville's domestic help. Thus, the convict is part of a lower class than Holmes, and therefore is not a worthy adversary. Stapleton, however, is an intellectual, and when his evil side comes out, his hidden nobility comes out as well. Once Holmes is handling an educated and noble rival, he begins to take things much more seriously. In this sense, Stapleton's character adds to the strong classist themes imbedded in this book.