The Red Rust of Death — Pierre Audemars

The celebrated detective M. Pinaud is sent to a remote village in the south of France to solve the mysterious death of the depraved and hated Marius Cujard. There, Pinaud meets the landlord of the village inn, a schoolmaster, the local doctor, and an outstanding priest. He finds out that each of the first three had good reason to murder Cujard, and that all four were seen entering and leaving the dead man's house on the night of his murder. But who was the killer?