Scott Dawson, 41, and his 72-year-old mother Carol denied murder but a jury decided they had brutally killed keen sportsman Gary Dean - who was autistic.
He was shot in the back with an air rifle before being beaten to death with a rock at about 7am on September 6 of last year.
His body was found in a ditch bordering the Dawsons’ land off House Carr Lane, Hood Green, near the Trans Pennine Trail by a passer-by who heard the victim’s mobile phone sounding at about 5pm - a call made by his worried wife, Caroline.
‘High concentrations’ of dirt found on the palms of Gary’s hands and his knees suggested he had attempted to crawl away early on in the attack before suffering fatal blows.
The Dawsons, who blamed Gary for damaging outbuildings and machinery over a four-year period, showed no remorse in court for their actions and opted against giving their own evidence.
They were sentenced on Monday having been found guilty last Friday following a five-week trial held at Sheffield Crown Court.
Judge Jeremy Richardson QC told Scott, of Allotts Court, Birdwell, that he would serve a minimum of 31 years behind bars, while Carol, of Stonewood Grove, Hoyland, will serve a minimum of 26 years.
He said: “You both fed off the venom within each other and this toxic mixture led to murder.
“You each played your part in encouraging and facilitating this with brutal determination.
“Gary Dean, a much-loved man who was vulnerable due to his autism, suffered terribly in the final minutes of his life. He was shot, attempted to flee and then was attacked for a second time.
“You both had a desire for revenge and neither could control your temper. You had murderous intent because you thought Gary Dean was thwarting your attempts to rent out your land for farming purposes, but he was doing nothing of the kind.
“What you did was deeply malevolent. It was truly wicked and cruel.”
The pair hatched a plan to kill Gary, which saw the ‘powerful’ air rifle being purchased by Scott - who Judge Richardson said delivered the attack - while his mother surveilled Caroline and effectively acted as a ‘lookout’.
On September 5, the day before the murder, Gary had started a fire in woodland bordering the Dawsons’ land and this acted as a ‘catalyst’ for them to press ahead, Judge Richardson added.
“There is a public right of way which cuts across your land,” he said. “You both reacted adversely if anyone strayed onto it and had unreasonable attitudes. You expressed a desire to take the law into your own hands.
“The fire on September 5 was the catalyst. You knew Gary’s wife’s work patterns and hatched a plot. You, Carol, laid in wait and reported that she had left. Gary was a man of almost ritual habit who always went running before breakfast.
“There was a clear intention to kill and he was brutally attacked. He scrabbled away but was pursued and you finished your murderous plot. It was a vicious death with both mental and physical suffering.”
Temporary Detective Chief Inspector Mark Oughton said he was pleased with the verdict and praised his team of officers and the Dean family for their help in securing two successful convictions.
“The judge did say justice had been done and I do believe that,” he said.
“I want to say a big thank you to the family, who supported the police over the 11 months of this investigation. I would also like to praise the police officers involved - it’s been a very difficult investigation as the court heard there was no direct forensic evidence.
“It was a very strong circumstantial case that had to be put together which ultimately led the jury to reach their guilty verdicts.
“Obviously the verdict is no consolation to the family but I want to praise how brave they have been and what dignity the have shown throughout.
“It was an absolutely brutal attack over a small dispute. There was some damage that had been done and the Dawsons believed Mr Dean had done that and it turned into a tit-for-tat.
“Unfortunately that went far too far and led to the brutal killing of Gary Dean last September. It was totally horrific.”