Becky's body was dismembered at the home of Nathan Matthews and Shauna Hoare
The stepbrother of Becky Watts has been found guilty of murdering her during a sexually motivated kidnap plot.
Nathan Matthews, 28, had admitted killing and dismembering the teenager but claimed her death was an accident and he was only trying to scare her.
A jury at Bristol Crown Court found him and his girlfriend Shauna Hoare guilty of conspiring to kidnap Becky, partly for sexual reasons.
Hoare, 21, was cleared of murder but convicted of manslaughter.
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Matthews, a former Territorial Army soldier, suffocated Becky at her home in Crown Hill, Bristol, on 19 February.
He transferred her body in the boot of his car and drove it to the home he shared with his girlfriend, less than two miles away, in Cotton Mill Lane.
In the days that followed, while police were treating the case as a missing person investigation, Matthews bought a circular power saw, goggles, masks and cling film which he used to carve up and package Becky's body.
The trial heard the couple had shared texts, just weeks before Becky's death, referring to kidnapping schoolgirls and teen-themed pornography was found bookmarked on a phone which they shared.
In court, Matthews confessed to watching pornography on a near-daily basis and the jury learned that a video about the rape of a teenage girl was found on a laptop at their home.
Police who interviewed Matthews and Hoare found a home filled with junk but the bathroom, where Becky's body was dismembered, was spotless
The trial heard that this sexual interest in teenagers, as well as a shared dislike of Becky, is what lay behind the plot to kidnap her.
When Matthews and Hoare knew police would be coming to their house on 24 February, as part of the missing person's search, they made arrangements to move Becky's remains to a shed at a friend's address, 80 metres away.
Two days later, forensics officers found blood stains on doorframes upstairs at Becky's home, among them was a fingerprint that would turn out to belong to Matthews.
After their arrest on suspicion of murder, he admitted killing Becky, in what he claimed was a bid to scare her that had gone "horribly wrong", and revealed where her remains were being stored.
But Hoare denied any knowledge of Becky's death and dismemberment, something the prosecution said was "ridiculous", as she had been with Matthews when Becky died and for most of the days that followed.
The jury took just over three hours to convict the pair, after hearing 21 days of evidence.
Hoare was also convicted of perverting the course of justice and preventing a lawful burial, charges Matthews, who gave his address as Hazelbury Drive in Warmley, South Gloucestershire, had admitted.
As verdicts were returned, Becky's father Darren Galsworthy and his wife Anjie - Matthews' mother - sobbed.
Matthews and Hoare remained emotionless.
Speaking outside court, Det Supt Mike Courtiour said: "Matthews and Hoare concocted a heinous plan to kidnap his stepsister Becky for reasons that they have failed to fully disclose, other than she occasionally left items on the floor which could have tripped up Nathan Matthews' mother.
"The body of evidence however suggested a more sinister motive related to their deep-seated hatred for Becky and their twisted sexual desires."
In a statement delivered by a police officer, Becky's mother Tanya Watts described her as "a beautiful, happy, funny, feisty, caring, loyal and witty girl" and thanked police and the legal team.
Sam Galsworthy, Becky's uncle, said: "Finally, justice has been done for our beautiful Becky. "
"For us as a family today is not the final chapter in this tragedy, but now we can at least begin the challenge to rebuild our lives."
Two other men - Donovan Demetrius, 29, and James Ireland, 23 - were acquitted of a charge of assisting an offender, related to the moving and storage of Becky's body parts.