Robert McCormick, 53, shot Peter Thompson, 59, in the head with a shotgun in a triple shooting on 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 happened at Mr Thompson's Meadowhead Farm near East Kilbride, Lanarkshire, last August. They were said to have been sparked by a feud over money between the killer and Peter Thompson.
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.
Now McCormick's will has revealed he left his £260,265.71 estate to his sister Margaret Ferguson, 53, to whom he sent a goodbye text message just before the shootings.
However, the document shows he had also racked up debts of £147,366.14.
McCormick, of Clarkston, East Renfrewshire, had not paid any council tax for four years and had arrears of £4,756.70.
He also had £127,142.40 outstanding on a mortgage and numerous credit card debts, including £7,275.27 on one HBOS card.
The former farmer had just £45 held in bank accounts at the time of his death and a shares portfolio worth only £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 his entire estate should go to Mrs Ferguson. She refused to comment when approached at her home in Newton Mearns.
Following the shootings, John Thompson managed to get into a van with a construction worker who was based nearby and was driven to nearby Auldhouse Arms pub to raise the alarm.
He was discharged from Hairmyres Hospital, East Kilbride, two days after the shooting.