The son of a working class Franco-Algerian family living just outside the Normandy city of Rouen, the teenager flipped between model student and aggressor as a youngster. He blipped on the radar of security services in early 2015, when he made his first failed bid to reach Syria.
Kermiche burst into a church on the outskirts of Rouen during morning mass on Tuesday with another teenage Islamic militant and killed the 85-year-old father at the altar, chanting in Arabic, before they were both shot dead by police.
"He was a loner. He was a troubled soul, he was all alone in his head," said a neighbour of the Kermiche family house in a leafy Rouen suburb where the 19-year-old was forced to live under a court surveillance order. "All he would talk about was Syria."
A judicial source said Kermiche received regular psycho-therapy and medication between the ages of six and 13, at which point he was sent to school for pupils with behavioural problems.
What role Kermiche's troubled background played in his conversion to a killer is not clear. Kermiche's radicalisation, however, was swift.
His mother told Swiss newspaper La Tribune de Geneve last year that Kermiche became "bewitched" by hardline Islamic ideology after militants attacked the satirical Charlie Hebdo magazine in Paris in January, 2015. Two months later, he made his first attempt to reach Syria to wage jihad.
Investigators are digging into the relationship between Kermiche and Abdel-Malik Nabir Petitjean, who lived in a French alpine town 700 km (440 miles) away from Kermiche, and how the two communicated before staging their attack.
Kermiche frequently communicated with scores of followers on Telegram, a private communication channel whose encrypted message system makes tracking chatter difficult for intelligence agencies.
In audio posts obtained by L'Express magazine and whose content was confirmed to Reuters by a police source, Kermiche told about 200 followers that going to Syria was no longer an option because of border controls, and urged them to launch attacks on French soil instead.
"You get a knife, go to a church, spread carnage, boom. You cut off two or three heads and you're done," he said.
Just hours before the attack, he posted another message saying "Download what's coming next and share it widely!!!!". He last logged onto the app at 9:46 am from inside the Saint-Etienne church, but he failed to post any video of the killing.
Friends said he would routinely try to indoctrinate them.
"Each time we said something to him he would come back at us with a verse from the Koran," said 18-year-old Redwan, a school friend of Kermiche. "He would tell us we had to fight for our Muslim brothers, that France was a country of infidels." He tried reaching Syria twice. The first time, he was intercepted in Germany in March, 2015, using his brother's identity card after his family reported him missing.
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