The Late Edwina Black (1951)
Taken from a celebrated stage play, this British film concerns the poisoning of the wealthy wife of a village school teacher in Victorian England and the quest by a Scotland Yard police inspector to uncover which of three suspects, who all stood to gain from the deceased's will, administered the poison. The director skillfully keeps one guessing by showing that each suspect was equally likely to have been the murderer only to subsequently sow doubt in the viewer's mind.
The ending has a twist that shows not only who killed the victim but why, and it has nothing to do with money.
9/10