His own parents were so frightened by his violence that they kicked him out when he was 16. Desperate, by the time he was 19, they dragged him to a psychiatrist, who prescribed an anti-psychotic drug, a tranquilliser and an antidepressant.
"There were the beginnings of a psychosis," the doctor, Hamouda Chemceddine, recalled in an interview in the Tunisian city of Sousse, looking over his notes from that visit in August 2004. "He wasn't someone who was living in the real world."
In France, he even created a Facebook page with an alter ego, listing his profession as a "professor of salsa dancing" and displaying a mock image of Nicolas Sarkozy, the former French president, in drag.
That man -- a 31-year-old delivery driver, Mohamed Lahouaiej-Bouhlel -- trained his violent tendencies on a crowd watching fireworks along the French Riviera on July 14, running over hundreds of people and killing 84 in a rented cargo truck during Bastille Day celebrations in Nice.
Since then, all of France has struggled to explain the single most murderous act yet committed by an individual since the country's wave of terror began. Was Lahouaiej-Bouhlel's rampage terrorism or merely the outburst of a madman? Or both?
The Islamic State quickly proclaimed him a "soldier". Yet Lahouaiej-Bouhlel's life -- pieced together in numerous interviews in France and Tunisia, where he was born and raised -- showed few signs of real radicalisation, and certainly no Islamic zeal.
Instead, it showed plenty of signs of verging psychosis and a hair-trigger propensity for violence by a man variously described as a drinker, a wife beater, a drug-taker and a chronic womaniser.
"He danced, he smoked, he ate pork. It was almost as though he wasn't even Muslim," Lahouaiej-Bouhlel's brother Jaber, 19, said outside the family home in Msaken, Tunisia. "He didn't even pray."
Rather, Lahouaiej-Bouhlel's life appears to show the ways in which the unstable and aggrieved have latched on to IS propaganda to shape their violent fixations and find permission to act them out.
In turn, the IS has latched on to them, declaring as its foot soldiers even individuals with tenuous ties to the group but long histories of personal and psychological troubles who are far from models of Islamic rectitude.
It's still unclear what led Lahouaiej-Bouhlel to his murderous rampage. But his killings have left the French authorities struggling to define the intersection of political terrorism and personal psychoses.
That intersection has spawned a new kind of killer, such as Lahouaiej-Bouhlel or Omar Mateen, who killed 49 people last month in a gay nightclub in Orlando, Florida. Their troubled and obscured interior lives led to hyperviolence difficult to anticipate or prevent.
The Paris prosecutor, Francois Molins, said Lahouaiej-Bouhlel had prepared his massacre for several months with the help of at least five accomplices, who were arrested.
Lahouaiej-Bouhlel's violent outburst was not that surprising to people in his old neighbourhood in Msaken, a bustling suburb of Sousse with a tennis club and a picturesque mosque. He was raised here as one of three sons and six daughters born to a hardworking and fairly prosperous farmer and property owner.
Instead, the neighbours felt shame, and many were resisting having him buried here, a local official said. No one mentioned his having the slightest allegiance to extremist Islam.
But many had stories of abuse by him, and knew to avoid crossing the bulky young man who skipped classes, worked out obsessively, bulked up on protein and flew into a rage at the drop of a hat.
"He would hit me when I was young," said a cousin, Zied Bouhlel, a lanky young man, wincing at the memory. "He did it as a joke, but it ended up hurting. Every time he saw me, as a joke."
Many recalled an episode when Lahouaiej-Bouhlel, driving a rental car past the local bathhouse, was rear-ended by another car. Both drivers got out, and Lahouaiej-Bouhlel proceeded to beat the daylights out of the other man.
"This was totally normal for him," said Hamila Hassen, who witnessed the confrontation.
Lahouaiej-Bouhlel had not been back in some four years, his brother Jaber said, adding that he had spoken with Lahouaiej-Bouhlel often by Skype, taking advice about clothing and food.
Distraught, he showed a selfie his brother had sent him from Nice's Promenade des Anglais just hours before the massacre occurred there, his brother's lips twisted in a slight smile.
Like many adolescents, Lahouaiej-Bouhlel appeared concerned over his physical appearance. But the young man sitting in front of him obsessed over it.
"He wasn't satisfied with the image of his body," Mr Chemceddine said. " 'I am ugly,' he said. 'I've got to build myself up'.
"He seemed strange to me.... He had an altered perception of reality."
Around 2009, Lahouaiej-Bouhlel moved to Nice, taking advantage of an arranged marriage to a cousin whose family lived there but was also originally from Msaken. The marriage was Lahouaiej-Bouhlel's ticket out of Tunisia, and he seized it.
By 2009, he had his French residency papers and a 10-year residency permit, which allowed him to work, according to a scan of the document. Soon he and his wife had a child.
After he got his documents, his treatment of his wife changed drastically, for the worse, according to neighbours.
"He married her to have the papers," said one, who gave her name only as Deborah and lived several floors below the couple. "He showed his real face afterwards."
She said she was close to his wife, and like her wore a hijab and had three children.
Lahouaiej-Bouhlel was interested only in women and drinking, she added. "My husband never saw him at the mosque."
By 2011 or 2012, he began to lead a double life, creating a false Facebook profile under the name Javin Bensucon.
He used the made-up identity to help him pick up women he often met salsa dancing, going to classes three times a week as well as going out for evening "soirees".
He kept his salsa world separate from his family life, and created a personality to go with it. In his Facebook profile, he was single, had gone to Cuba High and was from Bresilienne, a city in Haiti. He had 10 friends on the public part of his website, all of them glamorous-looking women.
He went out with multiple women and never told them he was married or a father.
Although his wife had two more children with him, each time hoping that would rekindle the relationship, it did not.
Sometime after their first child was born, he began to hit her. Sometimes he also struck his mother-in-law, said Jean-Yves Garino, the wife's lawyer.
"He was a narcissistic, perverted man, and he loved himself very much," Mr Garino said. The neighbour Deborah said that Lahouaiej-Bouhlel sometimes drank too much, and that his wife was often afraid of him. He would intimidate and demean her, she said.
She recalled a party with all of the neighbours one night, when he danced in a very seductive way with a 60-year-old woman in front of his wife.
Last year or the year before -- Deborah was not sure -- when his wife was observing the Muslim month-long period of fasting and abstinence during Ramadan, "he poured alcohol on her head to humiliate her", she said.
Around then, Lahouaiej-Bouhlel stabbed one of his children's toys with a knife. His wife became fearful for her children and sought a divorce, Deborah said.
He moved out, and occasionally gave her some money to support the children, but not much and not often, said Mr Garino.
In Nice, Lahouaiej-Bouhlel appeared on the police radar for theft and violence, most recently in January, when he got into a fight with another motorist and threw a wooden pallet at him.
Lahouaiej-Bouhlel received a six-month suspended sentence in March. By then, he may have already been fixated on carrying out violence on a grandiose scale, Mr Molins said.
But it was only in the two weeks before the Nice attack that Lahouaiej-Bouhlel began to research rather mundane Islamic topics, including information about Eid al-Fitr, the celebration that ends the month of Ramadan, Mr Molins said.
He searched the internet for information on the Orlando attack by Mateen, who had professed allegiance to the IS. But Lahouaiej-Bouhlel also searched for information on the recent killing of five police officers in Dallas.
Mr Molins said Lahouaiej-Bouhlel's photographs on his mobile, going back nearly a year, included ones showing crowds on the Promenade des Anglais.
Some images showed the crowd during the Bastille Day fireworks a year ago, another a crowd at a concert on the promenade, also last year.
Also included was an image of a January article from the local newspaper, Nice-Matin. It was about a man who purposely drove his car into a terrace cafe in the old port.