The question has come up, on several occasions: are there any mystery stories in which the butler really IS the culprit?
I managed to remember one in which the butler did it. Okay, okay, he’s really a rich man’s VALET is the killer… we Yanks who have no servants tend to use the terms interchangeably.
In G.K. Chesterton’s “The Miracle of Moon Crescent,” a short story featuring Father Brown, a millionaire is killed by his valet, a man he’d long ago “rescued” from life as a tramp. Far from being grateful, the tramp/valet held a grudge against his employer, who’d arrogantly posed as his benefactor.
Sorry, I should have provided the link.
Edgar Allen Poe’s “The Tell-Tale Heart”… granted, it’s not a mystery.
Americans at the time read British mysteries as avidly as they did American mysteries, probably more so. The British dominated the mystery field until about the 1930s. They would have read about the differences between butlers and valets regularly. In addition, mystery readers would have been wealthier and more educated than the norm. Books were expensive and difficult to get outside of downtown bookstores (although some libraries and lending libraries carried them). Only hardcover mysteries appeared in the 1920s, before Rinehart. Even the cheap hardback reprint editions that spread mysteries to a larger public in the decade before the paperback either didn’t start or were infrequent until the 1930’s.
It’s much more likely that butlers were used as murderers in British mysteries in the first place. There were more of them and they were more likely to be set in homes that had butlers. A story about a valet being the murderer is nothing more than a story about a valet being the murderer.
If the first half of your post is true, then I think the butler/valet is a natural choice for the target audience. Danger from an unexpected, trusted direction, someone with lots of opportunity, etc.
I don’t want to be spoiled so I’m not reading any replies, but… a Final Jeopardy clue of a few years ago traced the cliche to a Mary Roberts Rinehart story, and I recall one Agatha Christie in which the butler did it, except that as often is the case in Christie, he wasn’t exactly what he appeared to be.