But years of drug use and addiction left the 33-year-old living off government welfare and hand-outs from his mother after he squandered an elite private-school upbringing.
Stein was sentenced to life without parole on Monday for murdering Charlise Mutten, joining a small cohort of NSW's worst criminals condemned to spend the rest of their lives behind bars.
A jury convicted him of killing the nine-year-old by shooting her once in the back and then at close range to the face.
A jury convicted Justin Stein of shooting dead nine-year-old Charlise Mutten. (HANDOUT/NSW POLICE)
In sentencing Stein, Supreme Court judge Helen Wilson said there was nothing to reduce his culpability or help explain the "shockingly callous" crime.
Stein said he attended the elite Sydney all-boys schools Cranbrook and King's before dropping out in year 9, either due to his heroin addiction or having been expelled for abusing a headmaster.
After leaving school, he remained largely unemployed while maintaining a heavy dependence on opioids and developing a criminal record that included breaking and entering and larceny.
But it wasn't until 2016 that Stein saw the inside of a prison cell when he was arrested and sentenced to six years in jail over a failed cocaine importation plot.
For more than a decade he claimed he was in the process of setting up a wood and metal-working business, which he said he promoted through word of mouth.
However, Justice Wilson said Stein's only sources of income appeared to have been government welfare and support from his mother, Annemie Stein, who operated an antique jewellery business in Sydney's ritzy Queen Victoria Building.
The family also owned a sprawling Blue Mountains estate, Wildenstein, where Charlise spent her final night alone with Stein before being murdered by him somewhere on the grounds.
"Unlike most addicts, the offender has had the benefit of a materially advantaged upbringing," Justice Wilson said.
"His family has financially supported him for decades both in his living expenses and in gaining access to numerous rehabilitation programs."
None of Stein's attempts at rehabilitation succeeded and a penalty for failing or refusing a drug test in July 2023 suggested he had continued to use drugs while in custody, Justice Wilson said.
In one of the many lies Stein told in an attempt to avoid responsibility for killing Charlise, he said it was her mother - his former partner Kallista Mutten - who carried out the murder.
Justice Helen Wilson likened Justin Stein's tears in the witness box to "theatre". (HANDOUT/NSW POLICE)
Stein maintained the attempted deception throughout the days he spent on the witness stand, faking tears as he lied about watching Ms Mutten shoot her daughter.
Justice Helen Wilson likened the act to "theatre" and said Stein's willingness to lie in such a manner was one of the defining features of the case.
"Rather than demonstrating his anguish at the brutal slaying of a child he said he had been coming to regard as a daughter, it pointed to his complete lack of remorse for having murdered her," she said.
Noting Stein's "proven capacity to lie and lie extravagantly if he perceives it to be in his interests", Justice Wilson also rejected Stein's claims he suffered from mental illness and was abused at a young age by one of his mother's friends.
"I do not accept his self-reports of childhood trauma or mental illness as mitigating his offence," she said.
The only time Stein was witnessed by a health professional to demonstrate symptoms of his claimed schizophrenia was in 2022 when he appeared to speak to people in his cell when nobody else was present.
Justice Wilson said there was every possibility Stein feigned these symptoms after being questioned by police about Charlise's death.
"The only thing that can be comfortably concluded about the offender's mental health is that he has a substance use disorder," Ms Wilson said.
"An individual's decision to use illicit drugs does not mitigate his offence."
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
3°C