Bidon

The case you are describing closely resembles that of the Troadec family, whose killer was Hubert Caouissin. He murdered four members of his in-laws because he was convinced they had embezzled a family treasure of gold coins and ingots. After the killings, he transported the bodies to his home, dismembered them, and burned part of the remains in his property's boiler. The famous treasure was never found and its existence was never proven.