The pages of a will left by Derrick Bird's father provided an insight this weekend into the simmering tensions – and the substantial gap in wealth – between the taxi driver and the twin brother, who became the first victim of his shooting spree.

The document uncovered at the Probate Registry in London reveals how Joseph Bird, a civil servant for Cumbria County Council, gave David Bird a gift of £25,000 (€27,000) shortly before his death in 1998. Crucially, it made no such provision for other family members, including Derrick.

It was one of several perceived injustices, conspiracies and misfortunes – including a suspicion that his twin brother was conniving with the family solicitor to exacerbate his difficulties with a tax bill – that contributed to what became a murderous grudge.

"Rightly or wrongly, and we all think wrongly, Derrick thought his brother was out to get him," a family friend said last.

The will casts new light on more than 20 years of contrasting fortunes for the two men. Both were divorcees with a fondness for shooting game but, after a childhood in which they shared the same classroom, they spent their adult lives finding more to separate than unite them.

While Derrick (52) lived in a modest pebble-dash terraced house worth £90,000 in their native village of Rowrah, David, a mechanic and lorry driver who became a building contractor, had succeeded in buying a substantial farmhouse worth about £500,000 in rolling countryside some four miles away.

Divided by a will

The roots of the division that ultimately drove a despairing Derrick Bird to place his twin brother at the top of his target list may lie at least partially in their father's last will and testimony.

The document shows that Joseph Bird gave David the £25,000 in 1997 with a requirement that the sum be deducted from his eventual inheritance. The will said: "Having transferred money to my son David Bird absolutely, I direct that my son, the said David Bird, shall bring into hotchpotch upon the division of my residuary estate the sum of £25,000."

But there is no record of the money ever having been paid back, and while Joseph's estate was initially worth £200,000, debts and taxes reduced the eventual legacy to just £10,000, which was paid to his wife, Mary. Derrick, and the twins' elder brother Brian, received nothing.

Money had long been a sore point between the brothers. David, who has three grown-up daughters, seems to have been a canny investor, working hard to buy High Trees Farm, a substantial farmhouse and land in countryside outside the village of Lamplugh. It was in the master bedroom of this property that Derrick shot dead his brother at point blank range before dawn on Wednesday.

In 2004, David had sold a parcel of land to a developer to build four detached "executive homes", netting a significant profit on the eventual sale price of about £350,000 per property.

By contrast, the fates were less kind to Derrick, who, like his brother, left school at 16 to pursue a trade.

In 1990, the man who became a mass killer was dismissed from his job as a joiner at Sellafield nuclear power station, a vital economic lifeline in West Cumbria, where 67% of the region's workforce are or have been employed.

He was caught for a minor offence of stealing building materials and given a 12-month suspended prison sentence.

Around the same time, he separated from his long-term partner, Linda Mills, the mother of his two sons, Graeme (28) and Jamie (16). A neighbour said that Derrick would regularly be found outside Linda's new home, also in Lamplugh, at night time staring into the house.

He also became adept at salting away his earnings from his work as a self-employed taxi driver in nearby Whitehaven, amassing savings of up to £60,000, part of which, according to one friend, he kept hidden under floorboards in his home.

When Revenue and Customs learned of the sum in Bird's bank account, it seems formal steps were taken in recent months to recover an outstanding tax bill. The amount cited by Derrick Bird was £10,000 but sources said the sum could have been as high as £100,000. As a result, Bird was increasingly convinced that he was facing imprisonment of up to four years for tax evasion.

Mark Cooper, a fellow taxi driver, said: "All he said was that they had caught him with £60,000 in the bank. He just said 'I'll go to jail'."

As far as Derrick Bird was concerned, the problem was just the latest financial misfortune to assail him.

It emerged this weekend that the keen scuba diver had been counting on his share of the proceeds of his mother's home, sold recently after she suffered a stroke in 2003, to pay off his debts. Sources suggested that David had been reluctant to release the money or indeed surrender his share of Mary Bird's will in the event of her death.

In a statement, David Bird's daughters insisted on Thursday that there was no "feud" between the two brothers and their father had indeed sought to help his twin.

Nonetheless, Derrick Bird, his mood increasingly twisted by drinking binges and waves of paranoia about the motives of his family and friends, had decided that his two first victims were central to his predicament

A source told the BBC that Kevin Commons, the Bird family solicitor, had refused the taxi driver's entreaties in recent weeks to provide false testimony to Revenue that he felt would spare him prison. A pile of letters on the window sill of Derrick Bird's home, carrying the name of law firm KJ Commons & Co, bore testimony to the recent weight of correspondence between the two men.

Conspiracy to send him to prison

But, such was Bird's loosening grip on reality that he had come to believe the professional steadfastness of Commons was instead the result of a silent conspiracy with his twin brother to speed his passage to prison.

A close family friend said: "It was all about the Inland Revenue. He was being investigated and was paranoid his brother and the solicitor were colluding to get him in trouble with the taxman."

By the early hours of Wednesday morning, Derrick Bird's retribution for what he may have seen as a lifetime of fraternal inequalities was complete. David Bird's corpse was discovered by colleagues concerned at his failure to turn up for work at a nearby building contractors. The body of Kevin Commons lay on the driveway of his home. Their murderer was in his taxi, on his way to claim 10 more lives.