Out of universe, part of the reason is because Sir Arthur Conan Doyle got inspiration from a story about an English nobleman. He likely would have begun the story with the idea of using a spectral dog to take the blame and then written from there, resulting in the unclear reasoning behind Stapleton's actions that you're asking about.
I_am_1E27 t1_j61bc5g wrote
Reply to "Hounds of Baskervilles" question [Spoilers] by ricarleite2
Out of universe, part of the reason is because Sir Arthur Conan Doyle got inspiration from a story about an English nobleman. He likely would have begun the story with the idea of using a spectral dog to take the blame and then written from there, resulting in the unclear reasoning behind Stapleton's actions that you're asking about.
In universe, I have no idea.