Twisted killer Nicholas Prosper chillingly outlined his murderous intent in a 70-second video recorded just hours before he slaughtered his family.

Wearing a yellow bucket hat and a black T-shirt, the teenage fantasist calmly and dispassionately said he would take violent revenge against his 13-year-old sister Giselle over her actions in a computer game.

The video was one of several files posted online in which Prosper – who was also stopped by police on his way to carry out a school massacre - spewed his sick views, often signalling his obsession with children.

Prosper, 19, blasted his mother Juliana Falcon, 48, and siblings Kyle, 16, and Giselle in the head in Luton on September 13 last year before setting off towards his old primary school armed with a shotgun, a kitchen knife and a bag of ammunition.

In an echo of the Southport attack months earlier, the loner was consumed by a lust for extreme violence and harming children.

But he was apprehended by police just minutes before he could strike after officers spotted him lurking in the street around a mile from the school.

Prosper's chilling plans have now been revealed after he decided to change his plea to guilty at the last minute yesterday ahead of his murder trial next week.

It has also emerged that his disturbed ramblings were so depraved that he was kicked off a gore website – where millions of users share and comment on videos of people being killed – because of his warped stance on child abuse.

Prosper's content was uploaded under a pseudonym and featured his distinctive accent.

His videos have been deleted from YouTube in the wake of his appalling crimes, although the Mail has been able to view them.

They include a clip recorded hours before the massacre, believed to be filmed in his family's flat in Luton.

Prosper said: 'On June 30, my sister decided to make the incorrect choices on episode one of season one of The Walking Dead games, and for that her face will be mutilated further than necessary.'

Prosper said he was 'chosen' to protect a fictional character called Clementine in the zombie video game, in a sign he struggled to differentiate between the real world and make-believe.

The teenager spent so much time on the internet in his bedroom that neighbours had no idea he even existed – despite them being on friendly terms with his mother and siblings.

One woman told the Mail: 'I've lived here for 15 years and I never saw him once. Juliana and the other kids I saw, usually by the lifts.

'She would often say hello and chat, she was so nice. I saw all the other siblings but never Nicholas – ever. One lady who lives on their floor said she never knew he even existed.'

Prosper peppered online forums with his repulsive ideas, many relating to children and including the sexual abuse of dead bodies. 

His rants were so revolting he was banned from a gore website for repeatedly 'sexualising minors'.

One moderator told the Mail: 'There were multiple comments by him either in support of or arguing for child-adult sexual interactions that made it clear this was not in jest or a careless attempt to play devil's advocate.

'We do not know what sort of person Mr Prosper was, other than someone unwilling or incapable of following our site's rules and therefore someone we would not allow on our platform.'

In another video, Prosper sarcastically suggested that child sexual abuse was blamed for 'virtually every mental disorder and issue'.

Apologising for his 'slurred' speech, the teenager said: 'Who knows, maybe in a few years we'll see Aids, rectal cancer, increased chance of stubbing your toe, every single problem you face in life is because you touched a winky [penis] when you were six.'

Meanwhile the revelation that he was planning a school shooting sent shockwaves through the community as police revealed for the first time that Prosper's intended target was St Joseph's Catholic Primary School which he and his siblings had attended.

Police were alerted in the early hours on September 13 last year when a concerned neighbour called 999 after hearing screams in the family's flat at 5.30am.

Inside, officers found the bodies of the three victims who had been ambushed under the cover of darkness. But Prosper, who lived with his family, was missing.

Officers who flooded the area found the gunman a short time afterwards loitering in nearby Bramingham Road. There, he had hidden a loaded shotgun in bushes, along with more than 30 cartridges.

An investigation by Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit later uncovered substantial evidence that Prosper had been planning to commit a mass school shooting.

Officers discovered he had spent months fixating on extreme violence in the dark corners of the internet, spewing sickening views about child abuse and brooding on other school shootings such as the Sandy Hook massacre which left 26 dead in 2012.

His disturbed online ramblings in the months leading up to the murders revealed an obsession with children, paedophilia and a preoccupation with the Southport riots in the wake of Axel Rudakubana's attack just weeks earlier on July 29.

Prosper even claimed the disorder across the UK prompted by Rudakubana's triple murder at a Taylor Swift dance class last summer was 'beneficial' to Sir Keir Starmer.

Yesterday Prosper showed no emotion as he admitted for the first time that he had killed his family.

During a six-minute hearing at Luton Crown Court, the teenager, wearing a dark T-shirt and black-rimmed glasses, spoke only to plead guilty to three murder charges.

He also admitted buying a 12-bore Nikko shotgun, without a certificate, the day before the murders.

In addition, he admitted possessing a firearm with intent to endanger life and being in possession of a kitchen knife.

Prosper stood with his arms folded, biting his lip, during the short hearing. At a previous hearing, his estranged father Ray Prosper called out: 'I still love you, son. It's not your fault, OK?'

Afterwards, Detective Chief Inspector Sam Khanna, from the major crime unit, said: 'This was a truly tragic and shocking case in which three innocent members of the same family have been brutally killed by their son and brother.

'We are pleased that Prosper has pleaded guilty and will not put his family through the pain of a trial, and that he will soon face a significant jail sentence.

'What was subsequently uncovered during our investigation left no doubt as to his intentions to carry out an attack at a school, but fortunately Prosper was apprehended before he could cause any further harm.'

Assistant Chief Constable John Murphy added: 'We understand the deep shock this case will have on people within Luton and across the country.

'Juliana, Kyle and Giselle were brutally murdered in their home by someone intent on causing the maximum harm and distress on not only his family, but to the wider community.

'We are extremely grateful to our officers who stopped him and prevented him from going through with his plans to target a school.'

Maureen Murphy, headteacher of St Joseph's, said yesterday: 'We were devastated to learn of the tragic death of our former students Kyle and Giselle and their mother Juliana.

'All children from the family attended St Joseph's when they were younger and were a cherished part of our school community.

'At this time, our thoughts and prayers are with their family, friends and the wider community.

'We were very shocked to learn that St Joseph's was an intended target in this incident. However we would like to reassure our parents and carers that there is no threat to the school.'

Councillor Hazel Simmons, leader of Luton Council, said: 'The revelation that the perpetrator had plans to cause further harm in one of our schools is extremely shocking, distressing and upsetting; and we don't underestimate the emotional difficulty this news will have on children, staff, parents and carers across our school community.

'We will be asking schools to further review security measures in light of what came out in court today.'

Prosper was remanded in custody yesterday ahead of sentencing next week.