A teenager who had been classed as a “modern slave” has been convicted of murdering a child as part of a drugs feud.

Ayoub Majdouline, 19, was found guilty of “butchering” Jaden Moodie, 14, who was knocked off his moped and set upon by four youths who stabbed him nine times. The stabbing lasted seven seconds, damaged bone and punctured Jaden’s lung and liver.

Jaden’s family members, who had sat through horrific evidence including CCTV of their loved one being attacked, said “yes” with relief as the Old Bailey jury delivered a guilty verdict.

Jaden died in the middle of the road in Leyton, east London, in January 2019. In his last year there were six documented warnings to the authorities about his involvement in drugs, violence and gangs.

He had been dealing drugs in London for a gang called the Beaumont Crew, also known as Let’s Get Rich, and was likely to have been doing so on the evening he was attacked, the court heard.

Jaden was the youngest person killed in London’s gang violence this year, the Crown Prosecution Service said.

Jaden’s family had tried without success to stop him falling into a life of drug-dealing and told the Guardian of “begging” for help.

In interviews with the Guardian, Jaden’s family told of their pain. His older sister Leah, 19, said: “I’m not going to cry because I’ve got no tears left. Jaden has been appearing to me in nightmares. Having to sit through the murder trial has brought us right back to day one. Our family is dealing with trauma on top of trauma on top of trauma.”

Majdouline was acting for a rival drugs gang, the Mali Boys, notorious in the Waltham Forest area.

The background to the case reveals the perilous lives some British youngsters live, menaced by drug gangs who groom them and use them. Both youngsters were involved in county lines drugs dealing.

Majdouline was once classed by the National Crime Agency as a “modern slave” over his vulnerability to drugs gangs.

The prosecutor, Oliver Glasgow QC, told the jury: “This defendant was part of an armed group that went looking for a rival and, once they found him, the group chased down their target, the group produced their weapons and the group butchered him.”

On 8 January 2019 Jaden was riding a moped when CCTV captured a stolen black Mercedes ramming into him head-on. As Jaden fell to the floor, one person stayed in the car while four rushed out, armed with three knives, their faces covered.

Glasgow said: “Three of the males who got out of the Mercedes ran up to Jaden Moodie, who was lying defenceless and seriously injured on the ground … The three males embarked upon a violent and frenzied attack, repeatedly stabbing him with knives that they must have armed themselves with for the purpose of attacking him.”

Majdouline wore yellow rubber gloves as the attack unfolded. The gloves were later recovered by police and DNA tied them to Majdouline and the murder scene.

Both youths were known to social services and the police. Jaden had grown up in Nottingham and his family sent him to east London to try to get him away from trouble. Some warnings to the authorities came from his mother after she and her family received threats from gang members who came to the family’s Nottingham home looking for Jaden.

In January 2018, Jaden, then aged 13, was arrested in Nottingham with an air-powered pistol, a “Rambo-style” knife and cannabis. He was cautioned.

In October 2018, Jaden was arrested in Bournemouth, Dorset, on a drugs offence thought to be linked to county lines drugs dealing. He resumed dealing drugs in London after the Bournemouth arrest.

In November 2018, police came across a video on Snapchat of Jaden with an imitation firearm with the wording: “looool don’t fuck wid us please were here again.” He pleaded guilty to possession of the imitation firearm.

After he moved to east London, Jaden was excluded from a school there. It is understood he was previously excluded from school in Nottingham.

Jaden’s aunt Tesfa Green said her family had battled to keep the teenager safe: “My family radiate love. We adored Jaden. I could give you a list longer than my arm of our attempts to safeguard Jaden. Like when my sister painstakingly home schooled him after he was excluded from school in Nottingham, when he went to Jamaica to spend the summer with his dad so he could be a safe distance away from exploitative adults, when my sister was begging for support from children’s services.”

Dorset police said after arresting Jaden they returned him to London and made a “safeguarding referral” to Waltham Forest council.

The local authority in Waltham Forest, where Jaden lived towards the end of his short life, is producing a serious case review, due for release in February, which will consider if there were failings.

Jaden’s father, Julian Moodie, 51, said: “Sometimes my son appears in front of me and stares into my eyes. He died brutally at such a young age. It was supposed to be him burying me, not the other way round.”

Police are still looking for the four people in the car with Majdouline. He has refused to name them.

After the verdicts, Det Insp Dave Hillier said: “Our work is not over yet. We know that there were five people in that black Mercedes. Rest assured, our investigation is continuing around the clock and will continue to do so until all those responsible for Jaden’s murder are brought to justice.”

Majdouline had drug and knife-carrying convictions from 2016 and had come to the attention of the police and local authorities six documented times before the murder.

The 19-year-old, who lived in Wembley, north-west London, told the court he went back to drug dealing in 2018 after he failed to get benefits because it was “the only way I knew how to make money”.

Asked by his defence counsel, James Scobie QC, why he dealt drugs, Majdouline said: “Somewhere to stay, a little payment and, like, I didn’t have to spend money on food and stuff … I could buy weed and stuff.”

He said he carried a knife for his own protection. Majdouline was officially designated as a modern slave in 2018 after he was seen in Hampshire with a known drug dealer.

The NCA was concerned Majdouline was “being groomed by more sophisticated adult offenders”, according to a document produced for the trial containing facts agreed by the prosecution and defence.

One of the agreed facts was that he had been removed from his mother’s care because his stepfather abused him. Majdouline was abused in his next home and later went missing after being placed in various foster care placements.

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In January 2015 Majdouline’s father, Othamane, died after being struck around the head and stabbed by a man who had gone to buy class A drugs from his flat in King’s Cross, London, which was then set on fire.

The dynamics of gangs, criminality and desperation that played out in Jaden’s murder were foretold in a 2018 report by Waltham Forest council. It said gangs were motivated by the drugs business, with recruitment starting at as young as 10, and fuelled by greed, poverty and other social factors, and control was enforced by violence.

Majdouline will be sentenced on Wednesday. The mandatory sentence for murder is life imprisonment. He was also convicted of possessing a knife and had denied both charges.