The stepbrother of Becky Watts had no plans for what to do when his attempt to kidnap her went "horribly wrong", he told a jury.
Nathan Matthews, 28, denied plotting to escape justice with his girlfriend Shauna Hoare, 21, following Becky's death on February 19.
The pair, who allegedly share an interest in petite teenage girls, are accused of murdering the 16-year-old in her bedroom at Crown Hill, Bristol.
They allegedly suffocated 5ft 1in Becky, who weighed 9st, in a violent struggle before placing her body in the boot of their car and driving home.
Becky's body parts were found in a garden shed 80 metres from the couple's home on March 3.
Prosecuting, William Mousley QC suggested to Matthews that he spent the 10 days between killing Becky and his arrest formulating a plan with Hoare.
"You talked about what happened if you both got arrested for it and did you tell her out of your loyalty and love for her that if it came to it you would take it all on your shoulders?" Mr Mousley said.
Matthews replied: "No, I never said if we get arrested. I never said that, that would have been highly suspicious."
He said he "always had the worry" that he would be arrested following Becky's death but could not show it.
Mr Mousley asked: "That's why from then on since you tried to describe what happened upstairs in 18 Crown Hill on the 19 February as just a silly escapade that ended up as a dreadful accident?"
Matthews said: "I wouldn't have called it silly, I would have said drastic to shock and scare. It did go horribly wrong."
The prosecutor asked whether Matthews thought he was being clever and whether the problem was he was not clever enough.
Matthews sobbed: "The truth is I didn't plan for this, for what happened.
"In my head the plan was meant to go how it was meant to go and that's how I thought it would have went.
"Obviously once it went bad I had no plan. That should be pretty bloody obvious."
The former TA soldier denied the pair had planned to do a "disappearing act" after Becky's death.
"That wouldn't have solved anything," Matthews added.
The court heard Matthews and Hoare went to Becky's family home - where Matthews's mother Anjie Galsworthy lived - on February 19.
Matthews claimed he placed a 'kidnapping kit' of handcuffs, tape and a suitcase in the back of their Vauxhall Zafira before setting off.
After they arrived at the property, Hoare went to the garden to smoke a cigarette, which usually took her between 15 and 25 minutes, he said.
During that time, Matthews claimed he took the red suitcase out of his car before knocking on Becky's bedroom door.
He told the jury he expected the kidnap to take a "maximum" of five minutes due to "fear and shock".
Becky refused to get into the suitcase so he used a trick he learned in school to make people pass out, he told the jury.
Her breathing stopped and she had no pulse, so he zipped up the suitcase, dragged it downstairs and into his car.
Matthews told the court he suffered from fibromyalgia and struggled to hang washing but could lift a person weighing 12 stone.
"I didn't want to kill her," he wept.
Matthews and Hoare later drove home and he waited for her to fall asleep before unloading Becky's body into their front room.
Jurors were shown the red suitcase Matthews claimed Becky's body was inside but he refused to look at it, stating: "I don't want to".
He told the jury he moved the suitcase upstairs to his bathroom the following day while Hoare was at Crown Hill.
During that afternoon, he purchased a £80 Mac Allister circular saw, goggles, gloves and masks, and dismembered Becky's body in the bath.
"I closed my eyes, had the saw in one hand," he told the court, denying that it had made a mess.
No marks were found on the bath, which Matthews said was because he was holding the saw in one hand and Becky in the other.
"I wasn't slow, I was just rushing and doing what I had to do," he said.
Matthews said dismembering Becky and packaging the eight body parts in cling film, bags and tape was a lengthy process.
"I put some of it in the freezer and the other bit in the box," he said.
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.
The trial was adjourned until Friday.