A couple more noteworthy instances of the butler “doing it” come to mind.
Sometime in or around the late 1970s there was an above-average made-for-TV movie on one of the U. S. networks. I suspect it was meant as the pilot of a series which never got picked up, though it was way better than a good many police shows of the era. It concerned a not-especially-attractive middle-aged woman who was a police detective. A quirk of hers was that her office looked a bit like a greenhouse, with potted plants all around.
In the course of her investigation of the murder of a millionaire, she marvels at the many talents of the head of his domestic staff. Among other things, the man is a gifted ice sculptor. IIRC, he was played by Fritz Weaver. Unfortunately, Googling “Fritz Weaver” and “filmography” didn’t turn up the title of the movie, at least not before I gave up looking.
It finally turned out that Weaver was actually an extremely wealthy and cultured man who had only posed as a domestic servant so that he could become a member of the household and extract his revenge.
In one of those end-of-the-show scenes where the hero or heroine explains how the mystery was figured out, the woman detective suddenly stopped in mid-sentence and began laughing. The last line in the movie: “I just realized…the butler did it.”
Attention should also be given to the recent film Gosford Park. There a murder is committed twice (you’ll have to see it for yourself), and it turns out that the culprits, although not butlers, are well-respected and trusted servants.
There also comes to mind an old porn movie which was advertised with the slogan “the bulter did it–to everyone!”