The grievances and grudges that drove Derrick Bird over the edge

David Bird had no time to react to his brother's final visit. Before dawn on Wednesday, Derrick Bird entered the handsome white-washed house at High Trees Farm, climbed the stairs and entered his twin's bedroom. He then shot him dead at point-blank range.

Some 48 hours earlier, the two men had had argued about money. It was not a serious argument but it was a part of a jigsaw of events which pushed Derrick Bird over the edge. Outwardly an unobtrusive taxi driver, his problems – from a heavy tax bill to a messy divorce – had slowly fermented into a vengeful, dead-eyed rage.

Yesterday it became clear that, over three and a half hours, the 52-year-old divorcee's rampage through a 25-mile stretch of towns and villages in Cumbria had escalated from the methodical execution of a group of men he considered to have wronged him to a killing spree of terrifying randomness.

From the brother with whom he had always been "chalk and cheese", to the family solicitor, to – possibly – former colleagues at the Sellafield nuclear power plant where he was sacked for theft, Bird carefully rubbed out the lives of his targets after the psychological pressure of his mind had quietly reached boiling point.

Perhaps most disturbingly of all, it was a rupture he managed to conceal until the very last minute. Right up to Tuesday night, when he warned fellow drivers on Whitehaven's cab rank of a forthcoming "rampage", he was still the "Birdy" who paid £1 for his 85p carton of milk in his village post office and would never take his change. Here was a man whose sole apparent quirk was a passion for scuba diving and who always ran back into his tatty, terraced house in the village of Rowrah, 10 miles east of Whitehaven, to find a pound coin to pay for raffle tickets to raise funds for the local St Michael's Church.

Less than 48 hours before he unlocked his two guns and murdered 12 people, Bird went to watch a friend and neighbour compete at a motocross event, indulging his passion for motorsport. He later called in at the Hound Inn, the pub on the edge of the village of Rowrah where he was a regular.

Michelle Haigh, 40, who has run the pub with her partner, Peter Foley, for the last four years, said: "Derrick was in the pub on Monday night and was his usual self. He'd usually have two or three pints and a bit of a crack with people and then go home. That's what he did on Monday – he was just the normal Derrick."

Mr Foley, 44, added: "He was one of the last people to leave on Monday night. It was just the usual 'Good night Derrick' and off he went, as he always did. He was just a normal, sociable bloke."

What Bird, who had grown up in Rowrah and lived in the same house since 1993, did not tell his fellow drinkers was that hours earlier he had argued with his brother, a mechanic and fellow shooting enthusiast whose farmhouse home with sweeping views of the Lake District stood a few miles away in Lamplugh.

In a statement, David Bird's three daughters – Rachel, 28, Tracey, 26 and Katie, 19 – insisted last night that there was "absolutely no family feud" between the brothers, adding: "Our dad's only downfall was to try to help his brother."

However, The Independent understands the relationship between the two men was far from smooth. A former teacher at Ehenside Community School, where the two brother were in the same class, described the pair as "chalk and cheese", David being an outgoing, gregarious character compared with his quiet, reserved twin.

David Bird, a mechanic and digger driver for a local contractor who is understood to have moved into High Trees Farm in 2002, made a number of successful land deals, including an "executive development" of four detached houses on an adjoining parcel of land with a local developer in 2004, which netted a significant profit on the sale price of £335,000 per property.

Derrick had been significantly less successful financially than his brother. The taxi driver, who split up from his wife Linda in 1994, would work solidly throughout the Christmas period solely to finance his annual holiday to Thailand or the Red Sea – two of the world's best spots to further his hobby of diving.

A fellow taxi driver in Newhaven said yesterday that Bird had complained of a tax bill which he was struggling to pay and which he feared would result in legal action to make him bankrupt.

HM Revenue & Customs refused to disclose whether the gunman was under investigation, but it is likely that the "help" being sought by one brother from the other on Monday was a loan to stave off what Bird feared was impending financial ruin. Relatives dismissed suggestions that there had been a separate dispute over the will of Mary Bird, the ailing mother of the twins and their elder brother, Brian, 59, who also lives in Lamplugh.

A fortnight after he should have been celebrating becoming a grandfather after his eldest son, Graeme, and his wife, Victoria, had a son called Layton, it seems Derrick Bird's existence was instead increasingly overshadowed by questions of money – and fairness.

He grumbled privately about the growing practice of cab drivers on Whitehaven's Duke Street rank "queue jumping" by taking fares while at the rear of the line of waiting vehicles. Although official complaints had been made about the practice to Copeland Council, Bird did not feature among the complainants.

Mark Cooper, 45, a fellow driver, said the problem had been going on for "months" but Bird seemed to have taken particular umbrage. He said: "It's been going on for weeks and months; everybody does it. Obviously, with the financial pressures he had, and his mother being ill as well, it just got to him."

The extent to which Bird was psychologically troubled became clear last night when it emerged that he may have spent years self-harming.

The Independent understands that a post-mortem examination of the taxi driver found extensive evidence of self-inflicted wounds. An insider at an advice body helping sufferers said: "The police contacted us in relation to Derrick Bird. They say they have found evidence of years of self-harm on his body."

Ironically, one of the few visible signs that Bird's behaviour was becoming erratic was an act of generosity. On Tuesday afternoon, he turned up at the home of Graeme, 28, in Cleator Moor, about four miles from his own house, and handed over an unspecified but large sum of money, explaining that it was "for the baby". It was perhaps the first act in what the taxi driver saw as a reckoning.

Within hours, he had killed his brother, possibly as he slept, and then in the cool light of dawn driven to Frizington, where the Bird family solicitor, Kevin Commons, lived in a comfortable farmhouse.

It is testimony to the profoundly inter-connected nature of people's lives in Bird's killing ground that the woman who spotted the taxi driver lurking outside the solicitor's home at 5.30am on Wednesday, presumably moments after he had killed Mr Commons on his driveway, had gone to school with him.

Iris Carruthers, 49, who was at school with both Derrick and David Bird, said: "I was walking my dogs when I saw Derrick driving his taxi away from Kevin's farm. He had the window wound down so I shouted, 'Hiya lad, are you alright?' but I got no reply. He was in another world, he looked like he was in a daze. I just assumed he had been dropping someone off in the taxi. I was gobsmacked when I heard the news."

Tim Frost, a colleague of Mr Commons, confirmed yesterday that the solicitor had been "good friends" with David Bird. A large number of letters from the lawyer were also visible on the window sill of Derrick's home, possibly relating to a compensation claim for injuries received when the taxi driver was attacked by a teenage passenger in 2007.

With something akin to robotic efficiency, Derrick Bird had carried out his second killing and was preparing to end the lives of a coterie of "enemies". Darren Rewcastle, one of three cab drivers with whom Bird was alleged to have rowed about queue jumping on Tuesday night, was the next victim, left unrecognisable by the injuries to his face from his killer's shotgun.

In a chilling detail, Charles Brett, clinical director of emergency services at West Cumberland Hospital in Whitehaven, said at least five victims of the rampage had been shot in the face. "It appears [Bird] was firing out of a car window at head height. It's clear he was directing at the face and head," he said. However, he said he could not speculate as to whether this was Bird's deliberate intention.

In July 1990, Bird had left his job as a joiner for BNFL at Sellafield – by some distance the single biggest employer in the district – under a cloud after what police described as a "relatively minor incident of theft" and given a suspended 12-month prison sentence.

On Wednesday, after leaving Whitehaven, he drove via a circuitous route to the hamlet of Wilton and arrived outside the home of Jimmy and Jennifer Jackson.

Bird killed Mrs Jackson as she walked out of her house, turned around the car and drove on to kill Mr Jackson, leaving untouched a couple to whom he had been talking in the street. Mr Jackson had worked at Sellafield as a security officer at the time Bird was sacked, although the company yesterday insisted there was "no record" of a link between the men.

What will perhaps never be known is the point at which design in Derrick Bird's actions – deliberately hunting down a list of prepared targets – gave way to the callous hatred that saw him gun down a cyclist, a woman delivering catalogues and an estate agent in the most arbitrary fashion.

Before he took his own life at around 1pm, Bird's method turned fully to madness. Rugby-loving farmer Garry Purdham; 66-year-old Jane Robinson; former shipbuilder Michael Pike; mother-of-two Susan Hughes; mole catcher Isaac Dixon; and estate agent James Clark – all paid the ultimate price for falling randomly into the crosshairs of Bird's rifle.

As the full apparatus of the biggest murder inquiry in Cumbria Constabulary's history began its search for the answers as to why Bird acted as he did, one voice struggled to be heard. Mary Bird, who is understood to be suffering from cancer, was said to have a single request. Joy Ryan, a cousin of the Bird brothers, said: "She just couldn't make sense of it. She kept saying she wanted to talk to them, she wanted to talk to her sons."