He then beheaded him post-mortem.
The killing took place between when Clark sent a Facebook message at 8.31pm and when Mr Boyle called police at 8.39pm.
Later, while in custody, Boyle assaulted his lawyer, assaulted other custodial staff and removed his own eyeball.
Reports completed by a psychologist and two psychiatrists after Clark's death outlined Boyle's medical history.
Boyle had been in the care of mental health services since 2010.
In 2012 he was diagnosed with bipolar disorder after presenting with grandiose delusions which he had acted upon.
He was prescribed lithium and remained stable until 2015, when he had a further psychotic relapse, Justice Nation said.
In December 2016, Boyle was readmitted into psychiatric care and was discharged after six days.
In January 2017, a court order saw him assessed again after he had paranoid concerns, including believing his father was killing him.
He was mentally stable in 2017 and 2018; however, he was not engaging with his mental health team, Justice Nation said.
In October 2019, Boyle's father contacted his son's treatment team again with concerns his son was not taking his lithium medication.
His father was again concerned in late November 2019, when he saw his son going from normal to aggressive.
"It was reported in the month before 8 December 2019 he [Mr Boyle's father] had made some 40 phone calls to mental health providers to get Mr Boyle medically reviewed."
However, Justice Nation said reports showed Boyle was guarded about his mental health difficulties and was adept at hiding his auditory and visual hallucinations.
It was not until after killing his step-brother that Boyle was diagnosed as either being schizophrenic or suffering from a schizo affected disorder.
When Dr Justin Barry-Walsh interviewed Boyle between April and May this year, Boyle finally revealed how the voices he was hearing led him to kill his stepbrother.
He told the doctor his step-brother's death "was all down to the voices", which he had heard for three days. Those same voices told him if he didn't kill his step-brother his step-brother would kill him.
"I thought my brother was the devil because of the voices," Justice Nation read out.
Despite this acknowledgement, Boyle continued to deny it was him who had murdered his brother, he believed he had to kill him in order to release him.
"Mr Boyle believed his actions were morally correct."
Justice Nation told Boyle's and Clark's family, who were in court today, he was conscious this "tragedy must have devastated them".
He said Boyle's father had done all he could to try to get help for his son and the community should be sympathetic for what the family had been through.
Justice National ordered Boyle be detained indefinitely as a special care patient in a hospital until the Ministry of Health was satisfied he was no longer a danger to the public.
