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Alexander James William Merritt (23) received a term of life imprisonment for the murder of 51-year-old Karin Ann Ross, with a minimum non-parole period of 12 years.
The attack took place in the car park of Spotless Cleaning Services early on December 2, 2015.
The Crown case at trial was that there had been friction between the pair over Merritt's performance at work, which led to the explosion of violence.
Ms Ross was found dead in a pool of her own blood outside Spotless' premises in South Dunedin.
Shortly before trial in the High Court at Dunedin in 2016, Merritt was diagnosed with Autism Spectrum Disorder (ASD).
The matter was not raised with the jury.
Lawyer Tony Ellis took the case to the Court of Appeal last month claiming there was a miscarriage of justice as a result.
ASD threw into question whether Merritt could have formed the required "mens rea" (intention or knowledge of wrongdoing) to be found guilty of the crime, he suggested.
But Justice Forrie Miller disagreed.
"In some cases ASD may impair the person’s ability to foresee and appreciate the consequences of his or her actions; and if so, it may exclude the specific intent required for murder.
"We accept these submissions so far as they go, but they do not go nearly far enough.
"They identify a potential defence but they say nothing about its availability in Mr Merritt’s circumstances," the judgement said.
"The [psychologists'] reports . . . are not silent on the question of Mr Merritt’s capacity to form murderous intent.
"They suggest that he is well capable of doing so."
- Louisa Medlicott wrote that throughout his childhood Merritt learned to use aggression "when he has been thwarted or felt provoked, he has responded with aggression, sometimes significant aggression".
- Dr Justin Barry-Walsh stated Merritt was "immature" as well as "rigid and dichotomous in his thinking".
- Dr David Bathgate echoed these sentiments, noting that "it is clear… [that] at times when overwhelmed, [Mr Merritt] would become anxious and then become physically aggressive".
"The evidence establishes that Mr Merritt has learning difficulties and struggles in social situations, but it also confirms that he has a learned behaviour in which he responds with aggression to threats to his sense of security," Justice Miller said.
Mr Ellis was critical of trial counsel Anne Stevens QC but the Court of Appeal accepted she had discussed a "mens rea" defence with the defendant, which he rejected.
It also agreed that calling expert evidence about Merritt's ASD could have backfired.
"By establishing that he could form murderous intent and might with little provocation resort to violence, the evidence would tend to show that he was capable of committing an otherwise inexplicable crime and so detract from his chosen defence," the court said.
Since the appeal against conviction failed, Mr Ellis said there would be an appeal against the length of his sentence.