The stepbrother of Becky Watts described himself as an "emotionally unstable psychologically disturbed metrosexual Neanderthal" to police after his arrest, a court has 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 dismembered Becky - Matthews's stepsister - into eight parts, which were discovered in a shed 80 metres (260ft) from their home on March 3.
Bristol Crown Court heard Matthews told custody officers "I class myself as an emotionally unstable psychologically disturbed metrosexual Neanderthal" on February 28.
At that point, he had been arrested - along with Hoare - on suspicion of kidnapping Becky from her home in Crown Hill, Bristol, seven days earlier.
Both were later arrested on suspicion of her murder.
"I don't know how to explain it," he later said to police in an interview. "It is just like a caveman. I obviously have a hairy beard, but that's not anything to do with it.
"I would say when I am aggressive I am similar to a caveman. It is basically like a caveman, obviously like mentally undeveloped, not like a posh highly educated, anything like that.
"I react to my emotions. I have a fight or flight reflex where I say to people 'don't back a rat into the corner because they will attack first'. It is like an instinct.
"Obviously it has to be triggered by something."
Matthews said he could not quantify how regularly he lost his temper, adding that he was a "naturally stressy person".
He insisted he did not know if Becky was alive, if she had been hurt or where she was.
During the hour-long interview, Matthews described one occasion when he "snapped" at Becky but said it was not recent.
"I have never really had an argument with Becky," he said.
"Like I said, I snapped at her. I never had a proper argument with her, you can ask my mum [Becky's stepmother Anjie Galsworthy] or Shauna anything about it."
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.