A DIDCOT teenager locked up for trying to murder his sister with a claw hammer vowed to carry out a killing spree when released from prison.

The 16-year-old, who called himself Lord of the Flies and paid tribute to the school massacre at Columbine, became a Satanist after rebelling against his Christian parents.

And, as they sat downstairs, he attacked his sister by pretending to offer her a chocolate, before smashing her head 13 times and smothering her cries with a pillow.

Yesterday at Oxford Crown Court, the judge sentenced the teenager to at least four years in a young offenders' institution, but said he did not know when it would be safe to release him.

The attack left his sister needing 26 stitches and psychologically traumatised.

The teenager, who we are banned from naming, had predicted the exact day of his attack in videos, and written lists of family members, churchgoers and fellow school pupils he wanted to kill.

Sentencing, Mr Justice Saunders said he would serve a minimum of four-and-a-half years for attempted murder, but would not be released until medics deemed him safe to the public.

He added: “There is no determinate period that I could impose which would ensure at the time of his release he was not a danger to the public. He was brought up in a family with a very strong Christian faith. He rebelled against that faith.

“He developed an interest in the occult through listening to music which is described as ‘horrorcore’.

“His thoughts turned to killing people. This was no adolescent fantasy.”

The court heard the teenager had tried to find one schoolgirl’s address through Google Earth, and had planned to attack another boy while pretending to borrow a computer game.

During a week-long trial, jurors saw videos of the teenager burning bibles and blog extracts containing ultra-violent fantasies and images of corpses.

He daubed “hail Satan” and “I will kill you mum” on his bedroom walls, and on his website, he listed “shooting up schools like Columbine” as his occupation.

Mr Justice Saunders told the court that psychiatric reports indicated the teenager craved publicity.

He added: “Even when in custody after his arrest, he was writing of his intention to carry out a campaign of killing when released.”

Earlier, the teenager had told jurors his upbringing had been “confined” because of his parents’ faith and Satanism offered “a release”. He said: “(My sister) is, as far as I could tell, my mother’s favourite child. My intention was to hurt my mum through this.

“I felt assaulting my mum wouldn’t achieve that because I felt she would feed off the attention.”