Becky Watts's stepmother has insisted she still loves her killer son and will visit him in prison in a quest for answers.
Becky, 16, was brutally murdered in a sexually motivated kidnap plot devised by her stepbrother Nathan Matthews and his girlfriend Shauna Hoare.
Speaking after the pair were convicted by a jury on Wednesday, Becky's father and his wife Anjie Galsworthy, who is Matthews's mother, spoke of the pain of the "betrayal".
Mrs Galsworthy told Good Morning Britain her son had become a monster but insisted she still loves him.
"He's a different person, he's not the child I remember bringing up," she said.
"I still love him, I just find it hard looking at the monster he's turned into. He's totally different from the child I brought up."
She and her husband trusted the pair and thought police had made a mistake when they accused Matthews and Hoare.
"When they arrested them we still said 'you've got it wrong'," Mr Galsworthy said.
"We trusted them and the betrayal hurts so much.
"We loved them - they were all our kids even if they had different DNA. It didn't make no difference. We loved them, they were our family."
Becky suffered more than 40 injuries as she fought for her life in her pink and white bedroom.
Porn-obsessed Matthews, 28, is facing life in jail after being convicted of Becky's murder at Bristol Crown Court.
Hoare, 21, who was branded "calculating and cold-blooded" by police, was found guilty of manslaughter and trying to cover up the crime.
The couple, who fantasised about taking a teenage girl to the attic of their squalid home less than two miles away, had packed a kidnap kit in the boot of their Vauxhall Zafira.
They let themselves into her family's semi-detached house in Crown Hill, Bristol, while Becky was alone during the morning of February 19.
The truth about what happened to Becky, who was suffocated by Matthews, may never be known.
He spent the next three days dismembering her body into eight parts using a saw, a kitchen knife and a screwdriver.
The former TA soldier promised a friend £10,000 to store Becky's body parts - carefully wrapped in cling film, bags and tape and packed with cat litter and salt - in his garden shed.
Police searched the outhouse in the early hours of March 3, discovering the truth when a detective unwrapped Becky's severed right hand.
During a harrowing trial, the court heard how Matthews and Hoare swapped messages about kidnapping schoolgirls in the months before Becky's death.
A post-mortem examination found that after death Becky suffered 16 stab wounds and a long slash wound to her stomach.
Matthews and Hoare will be sentenced on Friday with Jaydene Parsons, 23, and Karl Demetrius, 30, who owned the shed where Becky's body parts were hidden. The pair admitted assisting an offender.
Matthews faces a mandatory life sentence but could die behind bars as the sexually motivated murder of a child can carry a whole life order.
Mr Galsworthy told the programme his daughter "had so many bruises on her, she fought for her life" and added Matthews has "no right to walk God's green earth".
He added: "I hate him, he's absolutely destroyed my entire family.
"I feel like an empty husk of a man now.
"I feel like I've let my daughter down because every dad should be able to fight for his family but I wasn't given that choice... they made sure I wasn't given that choice."
Mrs Galsworthy added: "No sentence could be long enough."