Becky Watts case: Nathan Matthews describes moment he killed stepsister Published duration 27 October 2015

media caption Nathan Matthews has admitted killing his stepsister Becky Watts but denies murder

A man accused of murdering teenager Becky Watts has told a jury about the moment he killed her.

Nathan Matthews admits killing his 16-year-old stepsister but denies murder and conspiring to kidnap her.

The 28-year-old told Bristol Crown Court his intention was to "shock and scare" her because of the way she treated his mother.

Mr Matthews said he had "thoughts about doing it for a couple of months, on and off".

He described going to her house, where he tried to knock the teenager out in a struggle to get her into a suitcase.

Mr Matthews said he was wearing a mask when he handcuffed her and put Sellotape over her mouth and eyes.

image caption Nathan Matthews and Shauna Hoare (right) are accused of murdering Becky Watts (left)

He told the court the teenager struggled as he tried to put her into the suitcase and described how her breathing slowed.

"I remember I couldn't hear any breathing, so that's when I thought something's not right," he said.

"So I checked her for a pulse and there wasn't a pulse. She didn't have a pulse. Then I shut the suitcase."

After putting Becky's body in the boot of his car, he said he returned to the house and slammed the door so his girlfriend and co-accused Shauna Hoare would think Becky had gone out.

He told the court he sat on the sofa thinking "act normal".

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In court: BBC Points West's Fiona Lamdin

Nathan Matthews is at times very distressed.

His mother Anjie was crying and her husband Darren had his arm around her, with the two gripping each other's hands in the public gallery.

When Mr Matthews is asked about his mother he sobs, leaning on the witness box by his elbows, hanging his head low.

He is squeezing a piece of paper and a tissue in his hands. Most of the time he is looking down, not giving any eye contact.

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Asked why he did not go to the police and admit what he had done, Mr Matthews said: "Because then everybody else would find out and would be hurt."

Becky went missing from her family home in St George, Bristol on 19 February and her dismembered body was found in a shed several days later.

Mr Matthews, 28, of Hazelbury Drive, Warmley, South Gloucestershire, also admits perverting the course of justice, preventing Becky's burial and possessing a prohibited weapon.

Becky's best friend Courtney Bicker, 17, told police in a recorded interview that Becky said Mr Matthews had "graphically" described how he would kill her.

Mr Matthews denied threatening to kill her and said: "No, I can't say for 100% [that] I didn't make a random joke about something, but I never threatened Becky."

He denied that he conspired with his Ms Hoare to kidnap his stepsister.

image copyright Avon and Somerset Police image caption Becky Watts' dismembered body was found in a shed several days after she vanished from her family home in St George, Bristol in February

Asked if he was controlling in his relationship with her, he said "she did mention it" but "it was not all one-sided" and "she had control over me as well."

He told the jury he suffered from anxiety and anger issues and said "if I am shown violence then I will show it back".

Mr Matthews admitted there was violence in his relationship with his partner and he had "scared her".

He said he had thrown her phone against a wall, shoved her head against a window while driving and pushed her into a lock on a door.

Ms Hoare, 21, of Cotton Mill Lane, Bristol, denies murder and conspiracy to kidnap, a weapons charge, perverting the course of justice and preventing a burial.

Two other men, Donovan Demetrius, 29, and James Ireland, 23, deny assisting an offender.

The trial continues.