Mysteries are at heart puzzles for readers to ponder as the narrative unfolds.
In order for the author to play fair with the reader, the culprit needs to be among the cast of suspects from relatively early in the story. This can be done well or poorly but revealing it to be a previously unintroduced character is far worse. It is similar to Anton Chekov's rule of minimalism for plays: If the audience is presented with a gun in tbe course of a play, that gun must be fired before the end.
https://en.m.wikipedia.org/wiki/Chekhov%27s_gun
Not including the antagonist is no different than presenting the protagonist with a task seemingly impossible for them to handle and then solving it with the miraculous intervention of a previously unrevealed agency; the classic Deus Ex Machina. Which comes from ancient greek plays that settled the issues of the story via divine intervention, literally lowering a platform holding the divinity of the day.
By current standards that is bad, bad form.
The classic genres (mystery, SF, Fantasy, thriller, western) and their subgenres have been around long enough to have evolved formalisms and rules of what constitutes a "proper" example of the genre. For mysteries, this includes giving the reader a chance to solve the mystery themselves. Preferably just a paragraph ahead of the protagonist.
What makes "the butler did it" a valid and classic genre solution is the butler is generally presented as an everpresent supporting character and traditionally subservient and passive. So, when the move first emerged it was truly shocking that a servant should be so active and self-directed. By now, it has been done to death which means resorting to that particular resolutionis either tured and hoary (if done poorly) or brilliant and witty (if done cleverly).
The key point is doing it right.