The moment you pick up that dagger you'll trigger a series of events that will end with you shut out of Castle Dimitrescu for the rest of the game in Resident Evil Village.
Again, I'll keep this spoiler free but at a certain point in the castle you'll find a coffin, and in that coffin is a body clutching a dagger.
The point of no return for exploring Castle Dimitrescu in Resident Evil Village comes when you find a certain knife.
