While Thompson's wife was unwell, the relationship between the two men became fraught, with arguments over unpaid rent and the state of the property.
It escalated to the point Riddle felt threatened and would avoid face-to-face meetings with his tenant.
Associates of Thompson also became concerned after hearing him state on several times that he would shoot Riddle if he continued to harass him.
A "go-between" was used to enable the exchange of rent.
Threats continued after the death of Thompson's wife in October 2017 following a lengthy illness.
In the weeks leading up to the murder, Thompson told someone he had a gun ready and would shoot Riddle without any hesitation if he felt harassed.
On the day before the murder, he again repeated he was going to shoot his landlord.
On February 6, 2018, Riddle phoned his tenant about rent arrears.
After going into town to withdraw money, Thompson invited Riddle over to collect the rent.
Thompson then called two associates - one of whom was the go-between - and told them Riddle was coming over.
"The gun is loaded and I've had enough of his s***, get up here now," he said.
Riddle collected the rent and was leaving when Thompson grabbed a 12-gauge double-barrel shotgun, confronting him at a side gate.
While pointing the gun at the 78-year-old he said "what did you say?"
The men were mere metres apart when Thompson fired the gun.
Maurice John Riddle died at a Normanby property. Photo / NZME
Riddle immediately fell to the ground critically injured.
Thompson walked back to his house and only returned outside when the two associates he had called arrived.
The pair rushed to try to help Riddle.
Thompson walked over and kicked the victim hard in the hip area saying "take that ... you got what you deserved".
Riddle died at the scene. Hundreds of people attended his funeral in the All Saints Anglican Church in Eltham.
When spoken to by police, Thompson admitted shooting Riddle and said that he "totally lost it".
At his sentencing, the judge believed the Crown was entitled to call it a "cold-blooded execution" for which Thompson had showed no remorse.
The judge ruled there were no credible grounds for contending a life sentence would be manifestly unjust.
The appeal was considered by Justices French, Woolford and Dunningham, who agreed a life sentence was "amply justified".
However, they decided the minimum period of imprisonment should be reduced.
Unlike the sentencing judge, the trio had the benefit of a further medical report about Thompson's delusional beliefs.
"We consider that a further reduction of 10 months is appropriate, resulting in a minimum period of imprisonment of 12 years and four months."