Reclusive cannabis addict De Zoysa was obsessed with weapons and watched violent videos on the internet.
He had been subjected to repeated beatings by his abusive father and appeared to be on the way to the family home with the loaded gun when he was arrested on September 25, 2020.
Asked if this was of significance, DAC Cundy said outside court: “Only De Zoysa knows why he was carrying a firearm on that night and whether he intended to use it or not."
De Zoysa told a psychologist he was concerned about food shortages during the pandemic and might need the gun to kill wildlife in order to survive.
He also claimed he began carrying a gun because he had been beaten up by drug dealers.
As well as making his own bullets for the antique Colt revolver he used to murder Sgt Ratana, De Zoysa also assembled a shotgun from old parts.
He told consultant forensic psychologist Dr Nigel Blackwood that he had worked on weapons in his garage in Banstead for six months before the shooting.
A pipe gun, dummy launcher, infantry rifle and various types of ammunition were found in his workshop when police raided it.
Mr Blackwood told an earlier hearing: “He spoke about ‘Mad Max’, the fact farmers had guns and that he considered ‘killing deer for food’ because the shops had ‘no food’ during the early stages of Covid.
“He had to change the bullets for his guns and ‘drilled them to fit’.”
Born in Croydon on July 17, 1997, De Zoysa grew up in Norbury, south London, with his parents and four siblings.
His Sri Lankan father Channa, admitted being a heavy cocaine user while his mother, Elizabeth, is an interpreter and a former Green Party local election candidate.
Diagnosed with autism aged 13, the highly intelligent child excelled academically at the Catholic John Fisher School in Purley, south London, where he got straight A's at A-level before studying mechanical engineering at University College London.
He told his trial he was bullied by fellow pupils who whipped him with chains because he was "tiny" and an "easy target."
De Zoysa had few friends but had a girlfriend for a short time.
Yearbook quotes from his classmates described him as "very clever", a "maths geek for the form quiz" and "good with weapons".
One former schoolmate said De Zoysa was reclusive, obsessed with weapons and "used to brag about being on the dark web".
The former pupil said he claimed to watch Islamic State films and bragged about seeing hitmen and guns for sale.
He said: "He also had a weird obsession for weapons and guns and would design them on school computers."
Channa De Zoysa was later convicted of ABH after he admitted hitting his son four times with the block of wood.
De Zoysa was spending £200 per month on powerful skunk weed at the time of the murder and had been arrested twice for fighting.
But he managed to hold down a fulltime job at HM Revenue and Customs working as a data analyst where he received bonuses because of his good work.
De Zoysa was considered by colleagues to be "technically adept" and one of the "few who could write his own coding scripts from scratch", the trial heard.
He moved out of the family home a year before the murder after his parents reconciled and his father moved back in.
His devices showed an interest in weaponry and violence across a spectrum of ideologies, including right wing extremism, Islamic extremism and homophobia.
De Zoysa was flagged to the Home Office’s Prevent deradicalisation programme two years ago but assessed as posing 'no threat."
