A brilliant scholar, football fanatic and a joker who always made people laugh - Sainey Marong never gave reason to suspect he would go on to commit a cold-blooded crime.

But with Marong now serving a life sentence for the 2016 murder of Christchurch woman Renee Duckmanton, his family in Gambia are left with grief, shame, shock and the question - what changed?

"Nobody would think he will ever do something, anyone who heard the news normally says is not him, who committed the crime," his brother Ebrima Marong told the Herald.

"What he did was unforgivable. He was not that brutal before, when he was in The Gambia. I don't know what really went wrong with him, to commit this heartless action."

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Sainey Marong, 33, was sentenced yesterday to life in prison with a minimum non-parole period of 18 years, in an emotional court hearing where he was shouted at to "rot in there, you maggot" by supporters of Duckmanton's family.

The Gambian-born butcher killed the 22-year-old sex worker in May 2016, after picking her up from Christchurch's red light district. He later set her body on fire.

At the High Court in Christchurch, Justice Cameron Mander described the crime as premeditated and predatory. He said Marong, who had arrived in New Zealand on a fake passport and was nearing deportation when he committed the murder, had been thinking about killing a prostitute for some time.

His internet searches in the weeks before the murder, which included kidnapping, local prostitutes, how to render a person unconscious, killing with bare hands, past murders of Christchurch sex workers, and necrophilia.

"That why I can't understand still...the searches he makes on Google," Ebrima Marong said. "I knew my brother so much we grown together since we were young and I know the type of person he is...he was a good person."

Ebrima, 28, said he and his brother were raised by their parents to be good and caring. They were born and raised in Gambia's capital city of Benjul as a member of the Mandinka tribe.

Marong was an excellent high school student, his brother said. He loved football and would refer to himself as "David Beckham". After school he began studying computer science and commerce at university, but dropped out to drive a taxi.

Ebrima described his brother as a funny person, who always made others laugh. He never smoked or drank, and always looked after others. He never wanted to argue, never quarrelled with anyone.

"I can remember there was a day I wanted to stand and fight with him even though he was my brother but he walked away and I was very shamed on myself," Ebrima said. "Honestly he is not a violent man."

In 2012, however, Marong fled Gambia, leaving his wife and three children behind. He crossed the border to Senegal, before making his way to Hong Kong, and then New Zealand on the fake passport in 2014. He worked legally for a time, but then authorities caught up with him and he was going to be deported.

The court heard Marong was a member of the United Democrat Party, and feared he would be killed because of his political ties. He had also witnessed the massacre of 14 students at a political rally in 2000.

Ebrima said he didn't really know why his brother left, only that there may have been some trouble with police. He had continued to send money back to support the family from New Zealand, Ebrima said.

News of the killing had come of a shock to his family, particularly his mother, who loved her son "very much", Ebrima said. They had been extremely stressed since the arrest.

"We are very, very sad," he said. "For both the victim and the accused. Human life is very important. I know Renee Duckmanton is a very innocent soul who don't deserve what happened to her.

He wanted to offer an apology on behalf of their family.

"I really know what he did was unforgivable and cannot really got away with it, is not fair."

He said while he knew what his brother did was wrong, he was still worried for him.

"He is my blood brother no matter how hard it is I can never hate him," he said.

All he and his family could do was have faith, and stay strong. He felt an immense pain for his brother, he said.

"I don't know if I will ever see him again."