Chau also spoke of his chilling idea with friends, a cousin and psychiatrists, but for legal reasons the judge, Mr Justice Michael Stuart-Moore, did not allow these conversations to be disclosed to the jury during his trial.

Criminologist Lai Ding-kee said serial killers were generally psychopaths who had no feelings for their victims, seeing them only as objects.

If they worked in pairs, they usually shared the same hatred or interests. When they found a common target, one took the lead while the other followed orders.

The nine-member jury found Chau guilty of the murders by an 8-1 majority but his co-accused Tse Chun-kei, 38, was cleared. Both had earlier admitted two counts of preventing the lawful burial of the victims by cutting up and cooking parts of their bodies.

Chau, a thin, quiet and reserved man, grew up in a well-off family and never needed to worry about money. With an above-average IQ of 126, he played piano very well and studied actuarial mathematics at an Australian university, but never finished the course.

Although his parents believed they had provided him with the best education, Chau grew to hate them, blaming them for his failures.

Friends said Chau entertained Nazi thoughts and even used Adolf Hitler's photo as the icon on his MSN account. His brother said he seemed to live in his own world and loved to manipulate people.

Tse's background could not have been more different from the man he befriended. He grew up in a close-knit, single-parent family, his mother the sole hard-working breadwinner for him and his sister.

An easy-going person of low intelligence, with an IQ of just 84, Tse failed all subjects in his school examinations. His sister said people could win his trust just by treating him to some food. A psychologist described Tse as a timid person who easily succumbed to orders under threat.

READ MORE: Son who killed and dismembered parents found guilty

But Chau and Tse had one thing in common - they were both failures in love. Chau only had some casual girlfriends on the mainland but none of the relationships lasted.

Tse had few relationships and attempted suicide when one girlfriend dumped him after spending all his money. He then married an Indonesian woman but learned on his wedding night that his wife had a husband and a boyfriend in her home town.

The pair first met in 2007 when they worked for the same company. They met again in 2010 after Tse's failed suicide bid. When Chau contemplated killing himself in September 2012, he spoke to Tse about their failures in life.

But Chau soon started to twist their conversations towards his wicked and murderous idea. The pair started discussing the plan via MSN messenger, with Chau dominating the conversation.

A month later, a shopping list of tools was created and possible methods of taking blood and dismembering a body were recorded in a notebook. They also prepared new telephone SIM cards as part of the plan.

They bought knives and other items in January 2013 and eventually lured the couple to Tse's flat at Tai Kok Tsui. The knives were used to murder the couple on the morning of March 1, Chau leaving the flat at noon that day.

It is believed the couple's bodies were cut up using tools including knives, saws and hammers in the following days. The couple's heads were stored in two refrigerators, and some bigger pieces were cooked and salted in plastic boxes.

Chau told police he and Tse acted in partnership - "each for one piece" - in the killing. He gave details of how the murders took place, such as Tse killing his mother then assisting him in the murder of his father.

But Tse denied taking part in killing the couple and dismembering their bodies. He claimed he only came to know the couple were killed when Chau showed him the chopped body parts. He suspected a man called "Ah Kin" had assisted Chau in the killing.

He said Chau had threatened to hurt his mother if he did not cooperate and he wrapped the human remains according to Chau's instructions, before dumping them into the sea on March 12.

Despite planning and executing the gruesome murder of his parents, Chau did not suffer from any mental illness, according to psychiatrist Dr Lui Sing-heung who met him 30 times. "He only had a personality disorder with narcissistic and psychopathic features," Dr Lui said.

He found that Chau was self-centred, totally unable to feel for others with a tendency to blame other people. He had high expectations of himself but only in his own fantasy world.