The stepbrother of Becky Watts repeatedly described how he would kill her in graphic detail in the years leading up to her death, a court heard.
Nathan Matthews, 28, is accused of murdering the 16-year-old with his girlfriend Shauna Hoare, 21, in a sexually motivated kidnap plot in Bristol on February 19.
The couple allegedly cut Becky - Matthews's stepsister - into eight parts, which were discovered in a shed 80 metres (260ft) from their home on March 3.
Courtney Bicker, 17, told Bristol Crown Court that her best friend was "scared" and "creeped out" after Matthews threatened to kill her on numerous occasions.
"He had described how he would kill her," Ms Bicker told the jury.
"I never heard him say this. It was just when they were alone together. She said it was numerous occasions and it scared her.
"I heard him say other things of a disturbing nature.
"They would have a normal conversation and he would randomly say that, from what she told me there was nothing that brought it up.
"Even though she thought it was a joke, she thought it was a bit serious. It creeped her out and scared her."
Ms Bicker said Becky had a good relationship with Matthews' mother, Anjie Galsworthy, and Hoare, but not Matthews.
"When I saw them they got along fairly well but sometimes - more from her - it wasn't that close," she said.
"On the surface it was one way but from what I heard from her it was a different way."
Ms Bicker said Becky first mentioned the threats in 2013, when they became close friends, and again in 2014.
Matthews described to Becky what method he would use but she never disclosed it to Ms Bicker, she added.
On one occasion, Becky told Ms Bicker that Matthews had threatened her "quite angrily" because she was doing something he did not like.
Matthews owed Becky £100 but the teenager felt unable to mention it to her stepmother Mrs Galsworthy, the witness said.
"He owed her money and he always ripped her off every time and she thought, 'this is my family, they shouldn't rip me off like this', and she's mentioned that to her grandma as well," she added.
Ms Bicker said Matthews was controlling of Hoare, who "got along quite well" with Becky.
"Becky said that Nathan wouldn't let Shauna cut her hair short and said he would buy her clothes to wear, it seemed really controlling but I don't think she realised it," she added.
But Margaret May, the grandmother of Matthews, later told the court that Hoare appeared to be the "dominant" partner in the relationship.
"She was much too young for him," she said.
"I didn't really approve because they did spend most of their time together and Nathan didn't do anything else.
"He became less interested in his appearance, he stopped working so that he could spend all the time with Shauna and he generally didn't seem that happy.
"She very often told him what to do. She seemed quite dominant in the way she spoke to him."
Mrs May, a mother-of-three, said Matthews told her he disliked Becky due to the way she treated his mother - her daughter - who had been diagnosed with progressive MS.
"It was usually just the way that Becky spoke to Anjie and expected her to do things that Anjie couldn't actually do any more, like clear up behind her, do the washing," she added.
"He was annoyed with her."
In a Facebook message posted in February 2013, Matthews wrote to Hoare: "I think my mum is lending Becky £500 because she wants a new TV, even though she's got a flat screen that works.
"You see. Becky runs the house and my mum's a d***head."
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, deny the charge.