Murder accused Kieran Greene told gardai that life with his partner's mother Patricia O'Connor had been "10 years of hell" and she "wanted everyone dead, down to the kids".
A court heard that Mr Greene said Ms O'Connor was not moving after he hit her on the head with a hurley in self-defence during a row, and he "let her die because my kids would be safe".
A Central Criminal Court jury heard evidence yesterday of interviews he gave gardai after he was arrested.
He said he believed she had tried to kill her husband by pushing him down the stairs.
He also claimed she would throw the children's Christmas toys out and also got rid of her husband's collection of World Cup milk bottles.
Mr Greene (34) denies murdering Ms O'Connor (61) at the house they shared at Mountain View Park, Rathfarnham, on May 29, 2017.
The grandmother's remains were found scattered over a 30km-wide area in the Dublin and Wicklow mountains between June 10 and 14 that year.
Mr Greene is on trial alongside three other people who have pleaded not guilty to impeding the investigation - his then partner Louise O'Connor (41), her daughter Stephanie O'Connor (22) and Louise O'Connor's ex-partner, Keith Johnston (43).
Mr Greene "handed himself in" at Rathfarnham Garda Station on June 12, 2017 and gave four voluntary interviews over two days in which he told officers he had killed and dismembered Ms O'Connor.
He said he had first buried her in a shallow grave in a cornfield in Wexford, but returned to move the body, could not lift it and instead cut it up and scattered the parts in the mountains.
He was arrested and interviewed four more times after he showed gardai the grave site and they saw what they thought was hair.
He told gardai he had sobbed after he dumped the body parts and felt "finally free".
Officers asked what he meant.
"My kids would be safe, nobody would have to fear her no more," Mr Greene replied.
On one occasion, he said, his partner's lips swelled up after taking a sip of coke. She was allergic to penicillin.
"I honestly think she was trying to kill us, but we couldn't prove it," he said. "She wanted us dead, one at a time, starting with her husband Gus. She wanted everyone dead, down to the kids."
On the night of the alleged murder, he said, as they struggled over the hurley in the bathroom, he hit her and she fell and "hit her head, I reckon".
He did not know if she was dead or not, but "knew she wasn't moving". Gardai said he had had a chance to save her but decided to let her die.
"I didn't. It wasn't planned. It was to keep my kids safe," he said. "I let her die because my kids would be safe."
He was shown CCTV footage from Mr Price in Tallaght, where he went to buy a petrol can, blades and tape on June 9, 2017.
He said the can was for the trip back to Wexford, but he did not use it, and the tape and blades were for decorating.
Co-accused Keith Johnston, who was with him, "doesn't have a clue why I bought them", Mr Greene said, adding that he "came with me because sometimes he drives the car".
Gardai later asked him if he got any help as it was a "big thing to move a body".
"No, as I said at the start, I did it all on my own," he replied.
It was put to him that his clothing, Patricia O'Connor's clothes and "what you used to cut up the body" had not been recovered.
Mr Greene repeated that he had thrown everything out in the mountains.
Gardai told him that, following a post-mortem examination of the body parts, the hacksaw he described to them "does not quite match". He could not explain this.
Former deputy state pathologist Dr Michael Curtis said the cause of death was blunt-force trauma to the head from a minimum of three blows with a solid implement and there was no sign of any defensive injuries.
The dismembered portions of the body had sharply-cut bone edges, he said, "the act of dismemberment most probably having been carried out with a powered tool such as a reciprocating saw".
In contrast, the severed wrists and hands showed more irregularity of bone ends, "raising the possibility that they were removed with a non- powered hand-held saw".
The trial continues.