A sports therapist has been found guilty of murdering a retired lecturer by shooting him with a crossbow at his remote island home in what police described as a “barbaric, medieval-style execution”.

Terence Whall, 39, targeted Gerald Corrigan, 74, as his victim adjusted his satellite dish outside his house in Anglesey, north Wales, in the early hours of 19 April last year.

The bolt passed completely through Corrigan’s body, causing terrible internal damage including bruising the heart. He was rushed to hospital but gangrene set in and he died almost a month later of multiple organ failure.

A ballistics expert and pathologist concluded the shooter must have been only 10 metres away, probably in the field in front of the house and hiding behind a wall.

Whall denied having anything to do with Corrigan’s death but police were able to establish that a Land Rover he used was near the victim’s home at the time of the shooting and on the previous night, when he scoped out the property.

Outside court, Corrigan’s partner, Marie Bailey, appealed for Whall to explain why he carried out the killing.

Bailey said: “To that sad, twisted broken soul who murdered him, I say if you have an ounce of humanity, any sense of decency then you would tell us now why you have done this.” She urged anyone who might know why Whall had carried out the murder to come forward.

Facebook Twitter Pinterest Gerald Corrigan was shot as he adjusted the satellite dish outside his Anglesey home. Photograph: Family Handout/PA

Corrigan’s daughter, Fiona, said: “The injuries caused by a crossbow are not designed just to kill, they are designed to mutilate. The particular weapon is designed for hunting to bring down game, and that is what my dad became: prey. We may never know why.”

She added: “The reason the news of the shooting was such a shock for us was that he was such a good man. Just an average bloke enjoying his retirement. He enjoyed a lie in, a nice cup of tea and reading books. He loved Laurel and Hardy films and photographing flowers and mountains. Our lives won’t be the same without him.”

DCI Brian Kearney of North Wales police said: “Gerald Corrigan was the victim of a barbaric, medieval-style execution in one of the safest parts of the UK. He was entirely innocent.

“Terence Whall believed he had planned and committed the perfect murder. There was no forensic evidence, no direct eyewitness evidence to the shooting and in fact no one saw him going to and from the scene.”

Peter Rouch QC, prosecuting, told the jury at Mold crown court it might never be known why Corrigan was murdered.

But the court heard that, before his death, Corrigan and his partner, Marie Bailey, 64, had allegedly been conned out of £250,000 by a man called Richard Wyn Lewis, who had previous convictions for fraud.

The court also heard there had been a row between Corrigan and Lewis over the growing of cannabis in outbuildings at the former’s home.

Jurors were told that Whall had been arrested at the Anglesey home of Lewis – who remains under investigation – on 31 May last year following an incident that the jury was told was a dispute over money. Rouch said Whall’s association with Lewis “may be of significance”.

Whall was arrested in connection with Corrigan’s death on 25 June. He initially lied to police and said he had been at his home on Anglesey at the time of the shooting, but later said he had been near Corrigan’s home because he was having a sexual encounter with a friend. The friend denied this.

In evidence not heard by the jury, the court was told a handmade garrotte was discovered in Whall’s car after his arrest, and his internet history showed he had bought piano wire online and googled “central nervous system neck injury”.

A punchbag, with holes that suggested it had been used for crossbow target practice, was seized from his home.

Co-defendant Gavin Jones, 36, was convicted of conspiracy to pervert the course of justice in connection with the setting fire of a Land Rover that Whall used to drive to the scene of the attack. Martin Roberts, 34, Darren Jones, 41, pleaded guilty to arson earlier in court proceedings.

All four will be sentenced on Friday.