Nathan Matthews broke down in tears as he told a jury he did not intend to murder his stepsister Becky Watts.
The 28-year-old started crying and slumped into the witness box holding his head in his hands as she started giving evidence during his murder trial at Bristol Crown Court.
Matthews and his girlfriend, Shauna Hoare, 21, are accused of killing the 16-year-old in a sexually motivated kidnap plot on February 19.
Bristol Crown Court has heard the pair targeted 5ft 1ins Becky due to their shared unnatural interest in petite teenage girls.
They allegedly murdered Becky in her bedroom at her family home in Crown Hill, Bristol, before driving her body to their home in Cotton Mill Lane, Barton Hill, Bristol.
Prosecutors say the couple dismembered Becky in the bath of the terraced property using a circular saw, before packing her body parts in bags, cling film and tape.
These were discovered in suitcases, bags and a blue box in a garden shed in Barton Court - 80 metres from their home - in the early hours of March 3.
Adam Vaitilingam QC, defending, asked his client: "Did you intend to kill Becky?"
Fighting back tears, Matthews replied: "No."
Mr Vaitilingam asked: "Did you intend to cause her really serious injury?"
Matthews answered: "No."
He then burst into tears, telling the court: "No, no... No I didn't."
Matthews then slumped into the witness box holding his head in his hands and loudly crying.
He then paused to recompose himself and took some sips of water before continuing to give his evidence.