This Poison Will Remain (A Commissaire Adamsberg Mystery)

In the south of France, three old men have died of the same spider venom. The recluse has a strong bite, but Inspector Jean-Baptiste Adamsberg is unconvinced that it's the cause of their deaths. His colleagues determine it an open-and-shut case, but the inspector believes a more sinister actor is behind the incidents.

Adamsberg soon learns that two of the three men had grown up in the same orphanage. At La Miséricorde Children's Home just outside Nimes, they had belonged to a gang of boys called the "band of recluses," known for their cruelty and violent behavior. As other members of the old recluse gang are bitten, the inspector moves closer to uncovering a larger truth, the tragedies hidden behind the walls of the orphanage.