Sobbing Nathan Matthews describes moment he killed stepsister Becky Watts

The stepbrother of Becky Watts sobbed uncontrollably as he recounted how he killed the teenager after trying to force her into a suitcase during a kidnap attempt.

Nathan Matthews, 28, hunched over the witness box, wept and twisted tissue in his hands as he gave evidence during his murder trial at Bristol Crown Court.

Matthews and his girlfriend Shauna Hoare, 21, are accused of killing Becky, 16, in a sexually motivated kidnap plot at her home in Crown Hill, Bristol, on February 19.

The jury previously heard the pair targeted Becky due to their shared unnatural interest in teenage girls.

After murdering her in her bedroom, they are accused of driving her body back to their home in Cotton Mill Lane, Bristol, and dismembering it in their bath with a circular saw.

Becky's remains were later discovered in a shed in Barton Court - 80 metres from their home.

During three hours of evidence, Matthews was intently watched by his weeping mother Anjie Galsworthy - Becky's stepmother - and Becky's father Darren Galsworthy.

Matthews, a former TA soldier, insisted Hoare played no part in Becky's death or dismemberment.

He went to Becky's home with Hoare on February 19 with a red suitcase in his boot loaded with a mask, stun gun, handcuffs, tape and black bags, which he planned to use to kidnap the teenager.

"I was going to scare Becky. I had the thought for a couple of months. I knew my mum was going out for the appointment... it was an opportunity," he said.

Matthews told the court his stepsister treated his mother, who has progressive MS, badly and left "trip hazards" around the house that could have left her "severely injured".

"It seems extreme but there had to be a shock and a scare to get through to her," he added.

Matthews claimed he took the suitcase out of his Vauxhall Zafira when Hoare had gone into the garden for a cigarette.

He took the suitcase upstairs to a spare bedroom to get ready and then knocked on Becky's door.

"The door was opened and straight away I used the Sellotape around Becky's mouth," he said.

"She turned around and I think I said something along the lines of 'As long as you do as you're told you are going to be fine'.

"She got down on to her knees and I got the handcuffs. I put the handcuffs on her hands, from behind.

"I was getting her into the suitcase when my mask slipped and my hands went over her eyes and I took the mask off and Sellotaped over her eyes."

Becky did not resist Matthews, who was using a deeper voice, until he attempted to put her in the suitcase.

"That's when she started like wriggling and resisting," he said.

"That's when I got my fingers on her nose. To restrict her breathing, so that she would pass out. Then I tried getting her back into the suitcase and saying 'Don't struggle, you will be released unharmed', and she was still refusing to get into the suitcase."

Matthews said he punched Becky once, causing her mouth to bleed, but did not want to cause her "a lot of violence and pain".

He then tried to do something he had learned in school "like strangling" by using his hands to restrict blood flow to her head.

"Obviously after that she stopped... stopped kicking," he said.

"That's when I moved her head and started moving her legs and pushing her into the suitcase."

Members of Becky's family left the public gallery in tears during Matthews's testimony.

"I remember I couldn't hear any breathing, that's when 'something's not right' and I checked her for a pulse," he wept.

"There wasn't a pulse there... she didn't have a pulse. Obviously then I shut the suitcase."

He told the jury he placed the suitcase in his car and behaved normally before driving home with Hoare. When she went to bed, he moved the suitcase into their front room.

The following day, he drove Hoare back to Crown Hill and left her there.

Matthews, who came up with a plan to dismember Becky hours after he killed her, then purchased drain cleaner, a circular saw, goggles, gloves and masks.

He carried the suitcase upstairs, placed Becky's body in the bath, cut off her onesie and green jumper, then dismembered her - but forensic experts could find no trace of Becky's blood in the bathroom.

"I just did it," he said. "I tried not to look. I did it and it was just like surreal, that's the only way I could explain it."

Matthews said he chose not to contact police after killing Becky as he wanted to protect his family.

"I was just trying to think of a way to protect them from the truth so they would think that Becky is fine, she's happy," he said.

He denied he intended to kill his stepsister or cause her really serious injury.

Matthews then burst into tears, telling the court: "No, no... No I didn't."

Matthews, of Hazelbury Drive, Warmley, South Gloucestershire, denies murder and conspiracy to kidnap.

He admits killing Becky, perverting the course of justice, preventing the burial of a corpse and possessing a prohibited weapon.

Hoare, of Cotton Mill Lane, Bristol, denies murder, conspiracy to kidnap, perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.

The residents of the Barton Court property, Karl Demetrius, 30, and his partner Jaydene Parsons, 23, admit assisting an offender.

Donovan Demetrius - Karl's twin brother - of Marsh Lane, Bristol, and James Ireland, 23, of Richmond Villas, Avonmouth - a work colleague of Karl's - deny the charge.