Nathan Matthews’s girlfriend Shauna Hoare was cleared of her murder but found guilty of manslaughter.
Matthews, 28, suffocated his 16-year-old stepsister while trying to kidnap her from her semi-detached home in Crown Hill, Bristol, on February 19.
He dragged her body into the boot of his Vauxhall Zafira and drove it to the home he shared with girlfriend Hoare, 21, less than two miles away in Cotton Mill Lane.
Over the next three days, Matthews bought an £80 Mac Allister circular saw, goggles, masks, bags, cling film and tape to cut up and package Becky’s body in the bathroom.
Packages containing eight body parts were stored in suitcases and a box then moved into a shed in Barton Court - just 80 metres away - in the early hours of February 24.
By that time, Avon and Somerset Police had launched the force’s largest missing person’s investigation, which culminated in Becky’s remains being discovered on March 3.
A jury of 10 women and one man at Bristol Crown Court returned verdicts after just three hours and 27 minutes of deliberations on six charges against Matthews, Hoare, and two men accused of helping to move and hide Becky’s body parts.
Matthews was found guilty of Becky’s murder, while Hoare was acquitted of the charge but convicted of her manslaughter.
Both were found guilty of plotting together to kidnap 5ft 1ins Becky in a sexually motivated plot fuelled by a shared desire for petite teenage girls.
Hoare was also convicted of perverting the course of justice and preventing Becky’s lawful burial by dismembering and hiding her body.
James Ireland, 23, and Donovan Demetrius, 30, were both acquitted of assisting an offender.
Friends and family of the popular teenager, including best friend Courtney Bicker, grandfather John Galsworthy, uncle Sam Galsworthy, brother Daniel Watts, stepmother Anjie - Matthews’s mother - father Darren Galsworthy and aunt Sarah Broom, sat in the public gallery wearing blue ribbons
When the jury forewoman stood up to deliver the verdicts, the atmosphere in Courtroom 2 was tense with anticipation.
As the first verdict was returned - that Matthews kidnapped his stepsister with Hoare - people in the upstairs public gallery began clapping and shouting “Yes”.
Mr and Mrs Galsworthy sobbed and clutched other members of the family.
Matthews and Hoare remained totally emotionless as all the verdicts were returned. After all the verdicts were delivered, Hoare dabbed her eye with a tissue.
Trial judge, Mr Justice Dingemans, said he would pass sentence on Friday from 10am.
A post mortem examination found Becky was suffocated during a “violent struggle” and suffered 15 stab wounds and a slash across her stomach after death.
Prosecutor William Mousley QC told the jury at the beginning of the trial: “She was suffocated, despite her fighting for her life.
“There followed a deliberate, carefully planned and grotesquely executed plan to cover up her killing.”
Mr Mousley said Matthews and Hoare planned to kidnap Becky and had brought a stun gun, handcuffs, tape and a large bag to her home.
“In addition to their apparent dislike of Becky, there is good reason to believe there was also a sexual motive behind the scheme, arising from a shared unnatural interest in attractive teenage females, and that a foreseeable conclusion to it being carried out would either be Becky’s death or serious injury,” he added.
Becky was alone at the home she shared with her father and stepmother on February 19.
Matthews and Hoare, who had been in a relationship for six years, arranged for a key to be left outside the property so they could let themselves in.
It is not known at exactly what time Becky was killed but she sent her final text to a friend at 11.03am.
The couple remained at the house - with Becky’s body in their boot - for a further eight hours before driving home.
They ordered a Chinese takeaway before using their phones to browse for television programmes and order a pizza.
Becky’s father reported her missing the following day and police launched an investigation.
Over the following days, Matthews and Hoare were captured on CCTV shopping for items including black bags, rubble sacks, rubber gloves and three rolls of clingfilm.
Mr Mousley said the couple spent three days completing “the lengthy process of cutting up and carefully packaging each individual parcel” and cleaning the aftermath.
After the hearing Barry Hughes, chief crown prosecutor for the South West said: “As we reviewed the evidence they gathered, it became clear not only that Nathan Matthews and Shauna Hoare had conspired to kidnap Becky Watts, but that their actions were motivated by their own sexual interests.
“Throughout the trial, Matthews maintained that he had killed Becky by accident during a bungled ‘prank’ to kidnap her. Having heard details of Becky’s injuries, as well as the cold and calculated way in which he attempted to dispose of her body, the jury has today found him guilty of murder.
“Shauna Hoare claimed that she knew nothing of Matthews’ plans or actions, that she had no involvement in Becky’s death or the concealment of her body. Again, the jury found the evidence did not support these claims and she has been found guilty of manslaughter.
“Unable to complete the task at hand without additional help, Matthews and Hoare recruited Karl Demetrius and Jaydene Parsons, both of whom will now be sentenced for assisting an offender for their unwitting part in concealing Becky’s body.
“Matthews and Hoare have never expressed any remorse for Becky’s death, demonstrative of the callous way they took a young woman’s life just as she was starting to plan her future.”