A 19-year-old was to go on trial Monday in Corsica for shooting dead his parents and 10-year-old twin brothers in 2009 in an act he said occurred in a trance-like state.
The accused, named as Andy, exterminated the entire family in August 2009 with his father's Winchester rifle.
He then left the house with a laundry bag stuffed with 2,500 euros ($3,200) he took from a safe as well as an expensive watch.
He was found by an uncle the next night on a nearby beach in shock, barefooted and wearing shorts.
"I didn't know where I was anymore. I didn't hear anything, and my vision was blurred. There was someone else in my place," he told investigators at the time.
He said his feet "dragged" him to the gun and he felt he "must kill."
Friends and neighbours told police that the family appeared normal and happy. Andy, who has been described as intelligent and athletic, has not given any explanation for his act.
Psychiatric experts are divided on Andy's responsibility for his actions that night.
Andy is being held in Borgo prison in Corsica, where he completed his baccalaureat in 2011. He plans to study science.
The trial is scheduled to wrap up on Friday.