Why would two young men thrash and abuse a three-year-old girl? Catherine Masters searches for an explanation, and for ideas to save other children from the cruelty endured by Nia Glassie.

Key Points:

Do you like Nia?" the cop asks the killer.

"Oaths [yes]," replies Wiremu Curtis, who was jailed this week for a minimum of 17 years, along with his co-murderer and older brother Michael Curtis.

The older Curtis barked like a dog as he was led away; the younger was unresponsive.

But Wiremu Curtis' "oaths" comment, in a video interview back when police were trying to figure their way through the lies surrounding what had happened to 3-year-old Nia Glassie could have been true, says his lawyer.

Craig Horsley thinks Wiremu, the boyfriend of Nia's 35-year-old mother, Lisa Kuka, did love Nia in his own way.

His client's video interviews were played during the trial and in them Wiremu's story spiralled out of control and he became increasingly upset.

"That second interview," says Horsley, "where he was tearful and distressed and upset - I don't believe that was an act. There may have been things he said that weren't true, that's a separate issue.

"The emotional reaction I think was genuine and on one level - and this is the paradox if I'm right - on one level I think he truly cared for and loved Nia and there was I think in that interview, a realisation of 'I've caused this, I've contributed to this'."

Wiremu came from a family rooted in violence and dysfunction. He had an older half brother who killed himself. After his parents separated he was raised by his mother and by a whangai aunt in Northland. His brother Michael remained with their father, William, who had joined Black Power at 14 and is soon to be sentenced for his own abuse of Nia.

Though Michael's lawyer would not speak to the Weekend Herald about his client, in court during sentencing he said that Michael was the product of a dysfunctional and violent upbringing.

His family had strong gang connections and he did not start formal schooling until the age of 12. He did not stay at school long.

Horsley is not offering excuses when he talks about Wiremu. But he thinks it is important to look at the circumstances of such families because without doing so there will be more Nias - and more Wiremus and Michaels.

Look at the circumstances leading up to Nia's death, for instance, he says. The three-bedroom house in Rotorua where she was fatally injured was crowded with adults and children. Before that they had all lived at another address, squashed into the one-bedroom apartment of William Curtis.

William Curtis' repeated abuse of Nia included wrapping a scarf around the little girl's neck and lifting her off the ground until her face turned purple.

His son Wiremu saw what was going on, says Horsley. Maybe such behaviour seemed normal. Most right-thinking people can recognise that hitting a child is wrong, the lawyer says. Wiremu however might be unable to differentiate between what is, and what is not, acceptable.

"If that's the environment you're brought up in and you are taught and are shown no different then I think it's a reasonable inference to say you don't know any different."

On top of that, Wiremu is described as intellectually slow. At the time of Nia's death he was 17. His cognitive age was that of a 12 year old. He is also virtually illiterate and not one for talking.

Horsley explains all this to offer some explanation. It's not rocket science to work out that unless society takes strong action there will be more Nias, he says.

Before Nia's death the country had reeled with the slaying of the Kahui twins. After Nia, a baby in Nelson was left brain damaged with skull fractures. There have been plenty of others.

Says Horsley: "We as a society haven't looked at putting abuse at the top of the cliff and the environments that give rise to Wiremu Curtis, I suggest, are prevalent in many communities in New Zealand today.

"If I'm right in saying that then tragically I'm saying, look, it's not a matter of if, it's a matter of when the next one happens."

Much has been made of the defendants' apparent lack of emotion in court. Horsley suggests that could be a reflection of their lack of cognitive function, of their lack of intellectual capacity. "You and I have emotions that probably plumb depths and reach peaks.

"They operate in a fairly narrow band of emotion I suspect." This doesn't mean they don't feel things, and doesn't excuse them, but it might help explain things.

"If you take Nia, for example, she's brought up in an environment where if she cries she gets hit, so sooner or later you learn not to cry. You learn not to show any emotion because any emotion may give rise to being harmed. If you're brought up in that sort of situation then arguably that just becomes your standard facade."

His own daughter goes to a decile two school where other students wag school.

"They're exhibiting all the signs that, in my experience, unless it's addressed now, there's going to be long term problems."

Why not fund a full-time truancy officer whose job is to get into the homes and get resources in where needed, he says.

If it costs $90,000 a year to keep a prison inmate, one truancy officer keeping tabs on 400 or 500 children who could introduce the services needed to a family, is cheap, he says.

The sentiment is shared by clinical psychologist Dr Ian Lambie who has experience in treating young people at risk and who has long called for more funding for the training of professionals who can help.

Lambie was on television last week, on a programme called The Outsiders, in which he tried to break through to 12 at-risk teenage boys heading down a path of destruction, violence and crime. Some of their early stories are horrendous, but for many New Zealand children and adolescents, sustained beatings and abuse are normal.

Not every abused child will end up a killer but such upbringings limit opportunities and the ability to make good choices, Lambie says.

"You get muddled that violence is okay, you are more likely to fly off the handle and explode and it's just at a level that the normal, everyday human being has no comprehension of."

People who watched the programme might also like to think about how at the same time they were watching, children in New Zealand were being abused.

"If you look at some of the perpetrators of the violence to Nia Glassie, those kids, you can identify them when they're five, six years old. There are no services, or few services for the kids aged seven to 12 years old in New Zealand and while you might think 'okay, yeah, well, we've got to deal with parenting programmes' you've also got to think 'well in five, six, seven years, they're going to be the parents'."

What they need is not one-off programmes but sometimes years and years of intervention and long-term follow up.

Services need to work together and teachers need specialist training and support to deal with students like these because there are hundreds, probably thousands, of them up and down the country.