Some of those who sat in the school hall with the blond youngster, before he took the surname of his infamous great uncle Ivan, barely remember him being there.

WATCH: The story behind David Auchterlonie's murder

"He was just the kid who sat around out the back of the school with the ADHD kids, you know, the ones who throw chairs around in class," one of his former classmates, Sam McMahon, said.

"He just wasn't known ... We're still trying to get our head around how that guy could do something like that."

In November 2010, Milat and Cohen Klein lured their friend David Auchterlonie, 17, to the Belanglo State Forest; tortured, tormented and then murdered him with a double-sided axe.

In the 18 months since, the young people of Picton, Bargo and Tahmoor, where the killers grew up, have spent a lot of time trying to understand how a once-reclusive high school kid became a cold-blooded killer.

The chillingly callous nature of the murder has been much described and discussed during the court hearing which preceded today's sentencing: the planning between Milat and Klein, the chilling 15-minute mobile phone "trophy" recording of the crime, and the bragging poetry Milat wrote afterwards in jail.

But the teenagers who used to hang out with David, Milat and Klein on the seats near the Bargo bakery on Remembrance Drive tell a different, more complicated story.

"I've known Matt [Milat] since I was about 12 because I was good mates with David and David was hanging out with him," Luke Ockers said.

"[Back then] he was just like the rest of us. He used to have fun and actually talk," he said.

Another friend, Dylan Smith, said: "It was Matt, David, everyone.

"Every couple of afternoons we'd all just go there and hang out at the seats until it got dark."

One of the striking points that comes through when talking to David's mates is how close he was to his killer.

The teenagers had known each other for at least four years before the November 2010 murder, spending many hours together at the various hang-out spots around Bargo, including, prophetically, the waterhole known as Mermaid's where Milat and Klein later tried to hide the murder weapon.

According to his mates, David was one of the few Bargo boys who stayed close friends with Milat as his personality darkened during the early years at Picton High School.

"As he started to get older he started to get weird; he just went really quiet," Mr Ockers said.

"He would sort of sit there not talking to anyone and playing with this flick knife that he carried everywhere. I didn't really talk to him that often as we got older, but I think David did."

According to the youth of Bargo, despite his penchant for carrying around a knife, Milat was not known for being violent.

"It wasn't like he got in lots of fights, you know, just the odd thing here and there," Mr Ockers said.

"He pulled a knife on David once because he wouldn't give him a cigarette, but he didn't do anything. I think he just thought about what he was doing."

However, Milat did begin to show a greater interest in his great uncle Ivan, including taking on his surname at the age of 14.

"I thought it was just for attention - to make himself out to be hard," Sam McMahon said.

He recalled that, a few years later when Milat had come out of his shell somewhat, he would talk about Ivan Milat openly.

"He'd sit there and go on and talk about the things that had happened with his uncle," Mr McMahon said.

"He would always want to have [cannabis] sessions in Belanglo. He was proud of it, just bragging about it, making himself out to be hard."

While no one has been able to explain why Milat's mind began turning to such dark places, there have been suggestions that he was unhappy at home.

His biological father, Peter Murphy, and his mother Deb Meuleman (nee Milat) split either before Milat was born or when he was very young. Ms Meuleman is the daughter of Ivan Milat's brother Bill Milat.

Milat spent most of his childhood living with Ms Meuleman and her husband Scott Meuleman.

While some have suggested Milat had a normal, happy upbringing, others say he did not get on with his stepfather.

During the recent sentencing hearing, the court heard that Milat had told a school counsellor he had been arguing with his stepfather and he "couldn't take it any more".

It was also suggested that Milat had taken on his great uncle's surname because he wanted to distance himself from his stepfather, but there was no evidence presented to support this.

Whether Milat's decision to change his name was motivated by pride in his great uncle or anger towards his stepfather, after the murder the teenager made a pointed reference to the deeds associated with his family name.

Court documents reveal that, the day after the murder, Milat told friends that he was doing "what his family does".

"Do you know what my family is known for? I killed somebody last night," he said.

During the murder, Milat accused his victim of telling people that he had stolen money from his mother and of "spreading his business" around town.

The Bargo boys and David's family say there were rumours at the time that Milat had taken some money from Ms Meuleman.

But David's grandmother Sandra Auchterlonie, said she did not believe this was the "real" motivation for the murder.

"I honestly believe he was emulating his [great] uncle ... to be a weekend killer," Mrs Auchterlonie says.

"I don't think they should ever be released. They're not right in the head. They need to stay inside."