The man, an only child, said he killed his parents in the southern city of Perpignan and ex-parents-in-law at Le Boulou, close to the border with Spain.
Officers confirmed that four bodies had been found at the indicated locations. The suspect handed himself over to the police on Sunday night after killing them over the weekend, prosecutors said.
“He reproached his parents… for being 'bad parents,'” deputy Perpignan prosecutor Luc-Andre Lenormand said in a statement.
The man described his ex-parents-in-law – with whom he had not been in contact with for around ten years – as manipulative and demonic, Lenormand added.
“All these years (the suspect) nurtured a fierce hatred of the victims. A hatred that led him to kill them on August 22nd and 23rd,” the statement said, without giving any details on how the victims died.
The man, currently in custody, risks a life sentence. He is due to be interrogated by an investigating magistrate ahead of being charged.
After his arrest, he was taken to hospital in Perpignan for treatment due to suspicions he had taken a large quantity of medication, the prosecutor said.