Robert McCormick, 53, shot Peter Thompson, 59, in the head with a shotgun at a farm.
McCormick also shot Mr Thompson’s son John, 36, in the legs before returning to his car and fatally shooting himself in the chest.
The shootings, which happened at Mr Thompson’s Meadowhead Farm near East Kilbride, Lanarkshire, in August last year, were said to have been sparked by a feud over money between the killer and Mr Thompson snr.
It is believed Mr Thompson had borrowed hundreds of thousands of pounds from McCormick for a business venture but had failed to pay him back.
McCormick’s will has revealed he left his £260,266 estate to his sister Margaret Ferguson, to whom he sent a goodbye text just before going on the rampage.
However, the recently published document shows he had also racked up debts of £147,366.
McCormick, of Clarkston, East Renfrewshire, had not paid any council tax for four years and had run up arrears of £4,756.
He also had £127,142 outstanding on a mortgage and numerous credit card debts including £7,275 on one Halifax Bank of Scotland card.
The former farmer had just £45 held in bank accounts at the time of his death and a shares portfolio worth £220.
However, he had two major assets, including his house in Clarkston which was valued at £150,000.
He also had a 50-acre plot of land on his family’s farm in Newton Mearns worth £110,000.
In the will, which was drawn up in 2003, he instructed that his entire estate should go to Mrs Ferguson, 53.
Following the shootings, John Thompson managed to get into a van with a construction worker who was based nearby and was driven to the Auldhouse Arms pub to raise the alarm. He was released from nearby Hairmyres Hospital in East Kilbride two days after the shooting.
McCormick held a gun licence and is thought to have owned several firearms. It later emerged his cause of death was a shotgun wound to the chest.
McCormick’s younger brother John committed suicide by shooting himself at his Glasgow home in 1984. He was 21.
Friends said McCormick had become depressed over money worries before the shooting spree and he feared bailiffs were coming to his house.
One said: “Robert came into a lot of money after his parents died and left him the farm.
“Peter owned a rubbish tip but, because he was bankrupt so many times, he borrowed money from Robert.
“Everybody told Robert to stay away from Peter and kept saying, ‘The tip is going belly-up, don’t give him any more’.”
Both Peter and John Thompson had been made bankrupt due to business difficulties.