Full-circle endings are always fascinating to me.
Sunset Boulevard ends where it begins- with Joe’s dead body floating in the pool. It isn’t until the moment that Norma fatally shoots him that the audience understands the narrator of the story has been dead the entire time.
On top of that narrative brilliance, Norma goes completely bonkers after that moment (though she was pretty crazy to begin with). We witness her final descent into madness along with a deceased Joe. She loses total control of her reality.
The films end was inevitable. The entire movie has been leading up to Norma’s breakdown. She lives for herself and her fame, so she must end with herself and her fame. She continuously contradicts everything that disagrees with her idea of fame, including the officer’s in the end. She sits looking in the mirror as though she’s preparing for a film. Norma pretends to be in her fantasy of fame for so long that the illusion becomes her reality.
Sunset Boulevard is about characters that believe their work to be better than it is. Joe is disillusioned because he believes himself to be a great writer, when really his work is just mediocre. Norma is unable to wake from her dream of fame that faded long ago. Both characters couldn’t continue living the way they were because what they were living wasn’t real. That’s why Joe and Norma both meet their demise, one physical and the other mental.