Stuart Hazell will be sentenced tomorrow after unexpectedly pleading guilty to the murder of schoolgirl Tia Sharp in August last year.
Hazell, 37, dramatically changed his plea on the fifth day of his trial at the Old Bailey.
According to his defence barrister, Lord Carlile QC, he made the decision because "Tia's family have suffered enough".
ITV News correspondent Juliet Bremner reports:
Speaking outside the courtroom today, Tia's father said that any sentence he receives will "not be enough" and called for Hazell to "serve his sentence [and] then be hung".
Tia was last seen at the house her grandmother shares with Hazell in Croydon, south London. Her body was discovered in the loft of the home after a week-long search by police.
Hazell is accused of sexually assaulting the schoolgirl before hiding her body from police.
In an exclusive interview with ITV News in the days following her disappearance, he insisted he did not know where she was and said she was "like my own daughter".
Lord Carlile said in mitigation that Hazell's decision to admit to the murder was a belated "act of remorse" and was "probably the bravest decision he's ever made in his life".
During the trial, the jury were shown an image of a child, alleged to be Tia, on a bed at her grandmother's home.
Jurors also heard how two memory cards containing "extensive" images of child abuse had been found at the home, as well as images and videos of Tia sleeping.
Hazell initially claimed in court that Tia died after falling down some stairs and that he had concealed her body because he panicked.
Today, his barrister told the court that Hazell "did not embark to kill for sexual or sadistic motivation," but that he accepted responsibility for her death.
Tia's mother said in her witness statement read to the court that she had not allowed herself to grieve yet and that she had placed her "ultimate trust" in Hazell.
Detective Chief Inspector Nick Scola described Hazell as an "violent and dangerous man who poses a significant risk to young girls".
He also apologised to relatives for the fact that police did not find Tia's body sooner despite two initial searches of the house.
A spokeswoman for the Crown Prosecution Service said that the strength of the case against Hazell had left him with little choice but to plead guilty.
Hazell will be sentenced at 10:30am on Tuesday.