I really like the Jekyll/Hyde theory. Frankenstein could have accidentally poisoned his brother, killing him. So he dedicates his life to finding a way to bring him back, only when he does, he’s not quite the same. We saw that Daniel still had parts of him remaining, and then he was also a monster. So the brother could be Jekyll while the Frankenstein’s Monster part of him is Hyde.
Loving this theory. Having Dr Frankenstein and Dr Jekyll being brothers, then Victor’s experiments turning his brother into Mr Hyde would be a perfect fit. Maybe the two doctors were working on some sort of research together, and that’s what led to one of them dying.