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The Corrections worker killed son Bradley, 9, and daughter Ellen, 6, as they slept in their beds in Dunedin last Wednesday night, in the home of his estranged wife, Katharine Webb.
"We're absolutely gobsmacked," said Livingstone's stepbrother-in-law Pete Scott.
"We don't even want to get our heads around those last moments ... it would just have been something out of a horror movie."
Livingstone's family, who live in Australia, had no idea the 51-year-old had taken the break-up of his marriage so badly, or that his wife had taken out a protection order stopping him contacting her or the children.
Neither did they know he had twice breached the strict conditions imposed by the protection order, or that he had told friends he wanted to murder his family and burn the house down with them in it.
Mr Scott, 57, watched Christchurch-born Livingstone grow up on the Central Coast in New South Wales, 80km north of Sydney.
He considered him a younger brother, often sharing a beer and barbecue duties, going fishing,and doing "all that man stuff" together.
There were never any signs of what was to come for Livingstone.
"There is absolutely no way in the world would you ever imagine in your wildest dreams that his life would end like this," Mr Scott said.
Livingstone, who had counselling and was taking medication after the break-up, turned up at Ms Webb's home on Kiwi St in the suburb of St Leonards, apparently carrying a petrol canister and letting himself in with a key.
Ms Webb ran screaming to her next-door neighbours, the Foots, telling them Livingstone had a gun and wanted to kill "my babies".
Chris Foot went round to try to talk sense into his mate but narrowly avoided being killed when he, too, was shot at.
Mr Foot found the children dead in their beds after Livingstone had taken his own life.
"I've read all the reports by the neighbours, and the guy who had his hair parted by the shotgun bullet, and that dude is just so lucky he's talking to us, but Ed wasn't a monster," Mr Scott said.
"I've know him for more than 30 years and watched him grow up. The media is making him out as a monster - he wasn't a monster. He was a bloody good bloke, and I really mean that.
"At the last minutes, hours, days of his life, he's just lost touch with reality and unfortunately it's come to this horrible, horrible ending."
Livingstone moved to Australia when he and his sister Suzanne were young children after their father, Duncan, a Scottish merchant seaman, went searching for work.
Mr Livingstone senior married Shirley Crease and Livingstone grew up with stepsister Karen Scott.
Livingstone lived a normal life, the Scotts said.
Out of school, he bounced around a few jobs - shop assistant, general labourer, according to Mr Scott.
More than 10 years ago he moved back to his homeland, and set up a new life in Dunedin. He met Ms Webb and they bought a bungalow in St Leonards.
They had two kids - Bradley and Ellen - and he had a job as an administration support officer at Otago Corrections Facility.
The Scotts lost touch with him until his father died "5-10 years ago" but reignited their ties at the funeral.
"Ed told us where he was living in Dunedin, by the water, so peaceful, and life was great. He loved his kids ... he couldn't have been happier," Mr Scott said.
"We were planning on heading over and spending some time with him and his family."
The Scotts heard his marriage had broken up but thought he was coping fine, living in Milton, and with a new girlfriend who manages a cafe in Christchurch.
The family is struggling to come to terms with the tragedy, Mr Scott said.
They don't know when Livingstone's funeral will be, but Mr Scott doubted they would come over for it. He believes his former friend "must have been crying out for help", but it wasn't available or he hadn't asked the right people.
"We'll just never know."