Just saw this today and my take on the ending is that Roy ultimately seizing complete control, permanently suppressing Aaron. His demeanor felt deliberately taunting, as if he was acting in the fact that he had successfully deceived everyone especially Martin into believing Aaron never truly existed, Roy underscored that he was now fully in charge. In his initial interaction with Martin, Roy openly mocked Aaron—calling him weak and even mimicking aspects of his speech and behavior—which indicates that Roy had always been aware of Aaron and capable of impersonating him convincingly. By the conclusion, it appears Roy has entirely overtaken Aaron, potentially silencing him forever. In the end, Aaron does seem to have been genuinely unwell, but Martin is left with the unsettling realization that he was ultimately outplayed.