Well, it probably does come a more of a surprise to any viewer who hasn't read the story; but when a chemist starts acting weird, engaging in "independent research," and finally disappears, it's pretty easy to forgive a detective who doesn't know what genre he's investigating to start with non-supernatural possibilities before moving on to "the victim was manipulated into resurrecting his alchemist ancestor who looks just like him, and who then killed and replaced him."

I suppose as a movie, it's not really that noir -- Curwen's quite evil, but no one else is really that bad.