I was wondering if the supposed split personality/dark side is not, in fact, '' a darker you'' and instead is a separate entity taking up residence inside, corrupting you as a means to its end goal.
I think it would be more interesting if it was an actual demon or otherworldly force of sorts instead of just a case of Dissociative identity disorder. Opens up the mythology of the story/world more.
Also, I remember a particular line of dialog the split personality said about picking up and starting over again in another town/place if it's plans failed here. Kinda sounded like he meant the MC was expendable and he'd simply possess someone else.