It is also a striking demonstration of the power of a single violent crime to continue causing pain more than a century later and half a world away. But when author Kate Summerscale first became aware of the story of boy-murderer Robert Coombes she had no idea that these wider themes would reveal themselves.

An award-winning writer and former journalist with two best-selling non-fiction books to her credit (The Suspicions of Mr Whicher and the Queen of Whale Cay), Summerscale was casting around for historical material that might form the basis of a new book.

She needed a strong narrative and, just as important, it had to be well documented to give her the required amount of detail and colour. It was while browsing London newspapers from the late Victorian era that she came across a promising true-crime story involving 13-year-old Coombes.

The teenager was living with his mother Emily and brother, Nattie, at 35 Cave Rd in the East End suburb of Plaistow. Their father was away at sea. In the early hours of July 8, 1895, Coombes entered his mother's bedroom and stabbed her to death with a knife bought specifically for the purpose.

Those bare, shocking facts were alone enough to bring notoriety to the killing but it was the behaviour of Coombes and his brother in the immediate aftermath of the crime that excited more than the usual level of interest at the time. While their mother's body lay in the upstairs bedroom, the pair watched the cricket at Lord's, visited the seaside and, apparently free of any sense of remorse or guilt, generally enjoyed life happily spending the housekeeping money their father had left behind.

The florid language of one local paper was typical of the wider reaction. "In the wildest dreams of fiction, nothing has ever been depicted which equals in loathsomeness this story of sons playing at cards in a room which the dead body of their murdered mother filled with the stench of corruption," said the Stratford Express.

In the same way, Summerscale's professional interest was also piqued. "It was that scene of them in the house with the body during that very hot summer in 1895 that first caught my attention," she says. "It seemed very weird and nightmarish and strangely innocent as well because the boys had been to the seaside and the cricket and the theatre. It was a very sinister but childlike scenario. I just became very intrigued by why they had done it."

At a practical level, she could find all the detail she needed. "Once I realised there was an Old Bailey trial and transcript and I had the names of all the witnesses and clues about the lives of the boys and the parents it felt like I could work with it and build a real picture of who they were, how they lived and what happened."

But there was also a nagging sense that there was more to the story and that something more intriguing lay beneath the terrible facts of the case. It was, she thought, almost too cold and ruthless for a 13-year-old boy. "There was something that didn't fit. I felt there was something else going on here."

After 10 days the boys were finally arrested, along with a family friend, John Fox, who was charged as an accessory after the fact.

With the rich trove of court documents, Summerscale is able to document minutely the trial of the three accused. The trial was a media sensation at the time, covered in prurient detail by the popular press. But despite all the commentary about the wickedness of the crime and the depravity of Coombes, the jury felt unable to sentence him to hang. There were questions about Coombes' mental state and whether he had been motivated to kill in defence of his younger brother who may have been physically abused by their mother.

Coombes was found to be guilty but insane and committed to Broadmoor psychiatric hospital, where he was to spend 17 years. Surprisingly, it turned out to be a relatively humane regime. "That amazed me as much as anything," says Summerscale. "There was compassion shown in that place to men and women who had done horrific things."

The extensive Broadmoor archive provided some detail on how Coombes had fared in the institution but the trail went cold for Summerscale at the point he was released. She turned to the internet and was astonished to find Coombes had travelled to Australia.

"I began googling with not all that much hope because I thought he had probably changed his name and could have gone anywhere," she says. "It was a real stroke of good fortune that I found this website where there are listings of graves in Australian cemeteries and even photographs of individual graves.

"It so happened Robert Coombes had kept his name and had a gravestone erected which straightaway gave me his date of death and I could infer his place of death and I knew where to go for more information."

The inscription on that gravestone in Coffs Harbour alluded to Coombes' war service and also bore a cryptic reference that he would always be remembered by "Harry Mulville and family". Even more remarkable, it turned out that Coombes had served at Gallipoli – but in a curious twist, was never a combatant, instead serving as a stretcher bearer and as a bandsman (he was an accomplished trumpeter).

"I found it astonishing to realise that the boy from the 1890s who had murdered his mother and been at the centre of a huge scandal had served with great honour as a soldier," says Summerscale.

"I think it is more than coincidence that he served throughout this incredibly violent and terrible war and somehow found himself in the position of always giving aid rather than inflicting violence."

If Robert Coombes' story ended there it would be remarkable enough but there remained one even more unexpected chapter for Summerscale to uncover. It concerned the Mulville family referenced on the headstone. She set about calling as many Mulvilles as she could find in the White Pages and ultimately made contact with Harry's youngest daughter. Even more surprising, it turned out Harry was still alive, aged in his 90s.

After the war Coombes had settled in the small village of Nana Glen, north of Coffs Harbour – next door to a teenaged Harry Mulville. Young Harry suffered terribly at the hands of his violent stepfather and, after one particularly savage beating, Coombes stepped in and brought Harry into his own home, caring for him as a son for the next five years.

To the Mulville family – and Harry himself – Coombes, who died in 1949, was their father's saviour,  permanently deserving of gratitude and respect. However, as Summerscale was to discover, they had no idea of Coombes' dark past.

"When I got in touch with his daughter I found out that the family held Mr Coombes in very high esteem but had no idea about his past and his childhood." Only briefly did she consider not revealing her research to the Mulvilles, instead concluding she had a responsibility to tell them.

The news has rocked the family. And it's a mark of how far it has affected them that Harry's youngest daughter, now 64, agreed to discuss it only on the basis her first name is not revealed.

"It has been quite a journey for me," she says. "We knew so little to start with about Robert. We knew none of this. I thought she [Summerscale] did an amazing job. I shed a lot of tears. I was very moved. She unlocked a lot of doors for me." Extraordinarily, the family is convinced Harry himself also knew nothing of Coombes' past. He has since died, but they decided unanimously not to burden him with the news.

"It's been a very big shock for the family," says the youngest daughter. "We felt we couldn't tell my dad because he was a very old man. I told Kate I thought the shock would probably kill him. But at the same time I had always wondered about Robert's past because everything seemed to be so quiet."

So how are we to consider the life of a man who killed his own mother as a child and then appeared to offer himself in the service of others almost to atone for his crime?

Harry's youngest daughter has no doubts. "Once I got over the initial shock, I thought, 'Goodness me, he has so turned his life around'," she says. "I don't hold any grudges against the man. Having read the story and everything he went through and how he spent his life after Broadmoor, I have more respect for him than I ever did regardless of what happened to him so long ago."

Summerscale, who has had perhaps more opportunity than anyone to consider the remarkable trajectory of Robert Coombes' life, also agrees.

"What Robert Coombes did for Harry is to my mind all the more precious and extraordinary given the beginnings of his life."

The Wicked Boy published by Bloomsbury Publishing, is on sale now.