While some films are made great by a certain amount of ambiguity in their endings, this is one that’s made great by its lack of it.
In the specific case of Leigh Whannell’s The Invisible Man, though, it’s simply wrong. The film’s firm perspective is one of the best aspects of its storytelling – creating a strong emotional bond with Cecilia and establishing the reality of the terrifying monster – and it’s also part of what makes the revenge-centric conclusion so satisfying.
Movies like Rosemary’s Baby, Shutter Island, and A Beautiful Mind perpetually keep the door open for thrillers where you are ultimately not meant to trust the protagonist, and it’s true that having a plot that involves a character’s sanity being questioned nicely dovetails with that established trope.