Ian Squire, 57, was among four Christian missionaries seized by a gang during a raid in the early hours of October 13 in the country’s southern Delta state.
Details of the medical charity worker’s death were initially withheld, but two of the other hostages held alongside Squire have now spoken of their horrifying ordeal.
Speaking to The Telegraph, Shirley and David Donovan explained that Squire was shot to death the day after the four Britons were captured, and just after singing a hymn.
Dr Donovan told the paper how the group’s spirits were lifted when the kidnappers returned a guitar and Mr Squire played ‘Amazing Grace’.
He told the paper: ‘It was the perfect song, and at that point things began to look not quite as bad.
‘But then, after Ian finished playing, he stood up, and a salvo of gunshots killed him instantly.
‘We didn’t see who did it, but it was obvious that someone in the gang had shot him. It was terrifying to see.
‘We jumped out of the shack and into the water as we thought they were coming for us next, but a member of the gang came and put us back in there with Ian for the rest of the day.’
They were held with Ian Squire and Alanna Carson as the four were working as missionary medics in west Africa.
The couple said they do not know why the captors murdered Squire, stressing that the gangsters refused to explain why they did it.
Squire and the other three hostages had been providing ‘free medical care and religious activities’ in the Burutu area of Delta state, said Chief Theo Fakama, from the local Enukorowa community.
The captives were freed after the kidnappers told them a ransom had been paid, and were met by two SUVs, with Mr Donovan saying the smell of the leather and the air conditioning ‘was like stepping from one world into another’.
The British High Commission and Nigerian authorities negotiated their release.
During their kidnapping, the group kept their spirits up by playing a version of BBC Radio 4 quiz The Unbelievable Truth, where contestants have to tell fact from fiction, with Mrs Donovan saying the game was ‘prosaic but comforting’.
They also reported that their captives – cultists whose gang was named after a local warrior god, Egbesu – were often seen drinking and taking drugs.
The Egbesu Boys are still being hunted by the Nigerian government, with their leader – Karowei Gbakumor – on the run.