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Paul Russell Wilson, who served 15 years for raping and murdering his 21-year-old girlfriend in 1994, stood beside Mr Bain at the alter during his Christchurch wedding on Friday, as he tied the knot with partner Liz Davies.
The killer was part of a three-man bridal party, which also included long-term Bain champion Joe Karam's sons Matthew and Richard.
Bain and Wilson met while working in the prison kitchen, a former inmate who shared a cell with Mr Bain said.
Wilson was already serving time for the rape and murder of girlfriend Kimberly Jean Schroder when Bain joined him in prison. He took the younger inmate "under his wing", the man said.
Nancy Schroder, the mother of Wilson's victim, said it made her "bloody sick" to learn the killer was out celebrating while the family continued to grieve.
The family had strongly opposed Wilson's parole and doubted his remorse.
"I heard they had shared a cell ... and now he's out celebrating. The murdering bastard. It makes you bloody sick."
Wilson was convicted and jailed in 1995 for the murder of Ms Schroder in Hokitika the previous May, a month before the Bain killings.
A 31-year-old sickness beneficiary at the time, Wilson visited Ms Schroder's flat and tied up her male flatmate. When his girlfriend came home he cut her hands during a confrontation, before cutting off her jeans and raping her.
He then stabbed her in the neck after she made comments about relationships with other men.
Wilson was sentenced to 15 years non-parole, though on appeal that was reduced to 13 years.
His parole was granted at his fifth time of asking in December 2010, with his behaviour described as "exemplary and faultless".
He was released in January 2011 into a residential programme included at least one year of intensive therapy. His release conditions also permanently banned him from visiting the West Coast or contacting the Schroder family, and he is subject to the "standard conditions" of parole for life, which includes recall to prison if he is deemed an "undue risk to the safety of the community".
Speaking to the Herald yesterday, Joe Karam said Wilson was Mr Bain's best and oldest friend.
The pair were two of the longest-serving inmates in their section of the prison and, though other inmates came and went, the two remained close.
"They sort of naturally became ... very close friends. The single person he [Mr Bain] has spent the most time with over the past 18 years is Paul Wilson."
Since his 2011 release, Wilson had reintegrated himself into society, Mr Karam said.
"He's done tremendous work to rehabilitate himself. He's got a job and qualifications."
The wedding itself was "outstanding", Mr Karam said. "It was beautiful in every respect. There was dancing all night. A lot of the guests said it was the best wedding they had been to."
His son Matthew, a 34-year-old Auckland lawyer, was officially designated as Mr Bain's best man, Mr Karam said, while his other son Richard, a 36-year-old Auckland businessman, rounded out the groomsmen,
"He [Mr Bain] thinks of them like brothers. It's not like they were pulled in because they were my sons. They've had a long association with him. They are really close friends."
Matthew, in particular, had visited Mr Bain in prison "virtually every Sunday" during his four years studying law at Canterbury University from 1998-2002, Mr Karam said.