Earlier quoted context omitted.
edit- after reading up on this, I'm not sure where you got your information from but everything I could find shows it to just be the common detective/mystery trope.
It just follows logically that historically some powerful families would have paid off a fall guy if necessary, and they could have used a butler in some cases. This could've been the origin of the term, before it even showed up in murder mysteries. Perhaps from the 1700s
The thing is, it doesn't show up in murder mysteries themselves (except only as a fourth-wall breaking meta-reference to the trope).
It originates with murder mystery readers -- as the name of a trope.