A coroner has highlighted a series of failings in the supervision in the community of a violent serial offender who murdered an 18-year-old in a random and unprovoked attack.

Nadim Bashir concluded Conner Marshall was unlawfully killed by 25-year-old David Braddon, who was subject to two community orders in the months leading up the killing.

Bashir told Pontypridd coroner’s court the “brand new” probation services officer supervising Braddon was “overwhelmed” by her caseload and strongly criticised the level of help she was given by her managers.

The coroner said the officer was struggling in an environment that was “chaotic and stretched” due to the impending implementation of reforms to the rehabilitation system.

Marshall, a store worker from Barry in south Wales, was savagely beaten by Braddon at a caravan park in March 2015 and died in hospital four days later.

Braddon took cocaine and valium before punching, kicking and beating Marshall with a metal pole. He stripped him naked and left him critically injured. When police went to Braddon’s home, they found knives and other weapons. He was jailed for life for murder.

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The inquest heard that Braddon was first convicted of an offence of violence at the age of 15. In the months leading up to the attack, he was subject to two community orders – one for attacking a police officer with his own incapacitant spray, the second for growing cannabis.

From the summer of 2014, Braddon was being supervised by an inexperienced probation services officer (PSO), Kathryn Oakley, who assessed him of being of medium risk.

Bashir highlighted that Braddon was one of 60 cases Oakley oversaw. Braddon missed a string of appointments and admitted that he was drinking and had come off medication that helped his mental health problems. Oakley accepted in court that if she had more time she would have made more checks and had a more investigative approach.

But the coroner said her shortcomings were not her fault but the result of her excessive workload and failure of senior staff to supervise her properly.

At the time, the controversial Transforming Rehabilitation (TR) government reforms were being implemented. Thirty-five probation trusts in England and Wales were dismantled and replaced with 21 privately run community rehabilitation companies (CRCs) to manage low- or medium-risk offenders, while the National Probation Service looked after those posing a higher risk.

The coroner said: “The circumstances in which she found herself, a brand new probation [services] officer, with woefully inadequate management and supervision structure … was not of her own making but rather that of both Wales Probation Trust and Wales CRC in the levels of staffing, caseload and structures they had in place for managing and supervising new PSOs.”

The coroner said Oakley was “failed” by Wales Probation Trust and added: “When employed by Wales CRC Ltd, her management and supervision was too little and too late.” He said: “Kathryn Oakley was overwhelmed with her caseload and workload.”

Bashir continued: “She was essentially left to her own devices, burdened as she was with a heavy and difficult caseload in an environment which was chaotic and stretched due to the impending implementation of the TR programme.”

The coroner criticised the evidence of some of Oakley’s seniors. He described the deputy head of Wales CRC, Terry Reddington, and team leader, Heather Nichols, as unreliable and evasive witnesses.

But he said Marshall’s murder could not have been foreseen or prevented. “To suggest that a more robust approach in the management and supervision of David Braddon would have … brought [him] back before the courts [is] purely speculative.”

Outside court, Marshall’s mother, Nadine, said: “The coroner’s findings have vindicated what we have always known to be true – that the supervision of David Braddon was not robust and the management systems were wholly inadequate. This was a direct consequence of the chaos caused by the privatisation of probation services. The coroner identified seven major failures. Had these failures not occurred we will never know if our son Conner would be here today.”

Ian Barrow, Director of National Probation Service Wales, said: “This was an awful crime and our thoughts remain with Conner Marshall’s family and friends.

“While the coroner found Conner’s death could not have been avoided, there is no doubt David Braddon’s probation supervision was not good enough.

“We have now taken responsibility for managing all offenders on licence in Wales from the community rehabilitation company and 800 more probation officers are in training across England and Wales which will help to improve public protection.”

The union Napo, which represents probation workers, said the case showed that its members faced unrealistic workloads that affected their ability to keep the public safe.

Deborah Coles, the director of the charity Inquest, said: “The Ministry of Justice and those responsible for the Transforming Rehabilitation programme must now take a long hard look at the consequences of their actions. The rushed privatisation of an essential service was done in the interests of profit over public safety.”