A resentful Derrick Bird was suffering from delusional beliefs which led him to carry out "vengeful, retaliatory fantasies" in shooting 12 people dead, a leading psychologist said today.

The mass killer was said to have blamed the rest of society for his failures but rather than killing himself alone he chose to cause grief to his community from the "vantage point of his own embitterment".

Dr Adrian West came to his conclusion after he was initially called in by police days after the shootings in West Cumbria to attempt to explain Bird's actions.

The consultant forensic clinical psychologist, who has assisted in many high-profile murder cases, went on to speak to 20 of Bird's associates, including his mother and ex-partner.

He found that on the surface Bird was an "ordinary man", described by everyone as quiet and passive, but in reality was someone who accumulated grievances and never forgot them.

Giving evidence at the inquests into the deaths in Workington, Dr West said he thought the gunman targeted his own twin brother David and his solicitor Kevin Commons to get his retaliation in first over his mistaken belief they were conspiring against him.

He then carried out "vengeance" against the taxi drivers who he thought had humiliated him before committing "dreadful violence" against random strangers in a bid to achieve notoriety on the morning of June 2 last year.

Dr West emphasised though that Bird, 52, knew what he was doing as he deliberately chose his victims along his driving route and made conscious decisions not to harm certain people.

He said: "It is my opinion that by the time of his death, Derrick Bird appeared to be a bitter, resentful and depressed man, blaming the rest of society for his failures.

"He appeared to be suffering from a delusional disorder which disinhibited him sufficiently to enact vengeful, retaliatory fantasies.

"Derrick Bird had the means and the knowledge to kill himself alone relatively easily in a diving accident or with a firearm. In causing grief to his community he chose from the vantage point of his own embitterment to reaffirm himself, believing that people would never forget him.

"He thought that by doing as much damage as he did he was achieving some kind of notoriety."

Dr West said Bird's mother, Mary, had told him that when growing up Bird kept his troubles to himself, did not argue with anybody and never lost his temper.

"She described that apparent self control as positive," he said. "I am not so sure as the years go by about how positive that was for him."

Bird was not without a social network with a close friend of 30 years or more and a number of drinking partners but some of the latter were superficial friendships, said Dr West.

All described him as quiet but the psychologist said that was because Bird "did not have a sophisticated emotional understanding of himself or other people".

He said: "What is likely is we have got a man who was accumulating a lot of grievances against him and he did not forget them.

"His point of reference was only himself. He just had his own point of view and his own view was increasingly resentful, hostile and ultimately led to an outpouring."

A culture of backbiting and wind-ups had developed at the taxi rank in Whitehaven where Bird worked in which everyone was queue jumping and undercutting each other in times of hardship, Dr West said.

"Derrick Bird probably did not roll with the wind-ups in the same way others did and probably took them to the heart because of the way he was," he said.

He said Bird would have thought it "particularly shaming" when some drivers told customers: "Don't get in his taxi, he stinks."

Bird had convinced himself that his brother and Mr Commons were setting him up following the meeting he had with an accountant to discuss his ongoing tax problems in which he owed up to £25,000.

That led him to believe he was going to prison and his life "was over".

He had spent years hoarding his cash and did not like letting go of his money, the inquest jury was told Dr West said Bird left his home in Rowrah on the morning of June 2 knowing that he would never return.

Dr West said the fact that Bird had left the deeds to his house out on the kitchen table was significant.

"There prominence seems to be very deliberate and while there was no suicide note the positioning of this clearly seemed to be an equivalent," he said.

"I saw Derrick Bird as having made the decision to commit suicide and that he knew when he left home he would die."

He added it was also curious that Bird had uncharacteristically not booked in his taxi for an MOT due on June 7 and had not submitted his taxi registration documents which were also due to be updated.

This probably indicated a degree of pre-meditation of what was going to happen, he said.

Dr West visited Bird's home as part of his studies and found that his back kitchen, toilet and bathroom were "filthy".

His bedrooms were described as "absolutely chaotic" and he thought it unlikely he slept in the beds near to the time of the shootings.

"I think he was increasingly become preoccupied to the point where he was not sleeping," he said.

The discovery of a stash of hardcore pornographic DVDs led to the trawl through the internet history on his computer where images of extreme porn were found.

Dr West said this was "not common" and indicated some "deeper resentment against women generally".

He said the evidence showed he made discriminate choices as to who he shot and how he shot his victims on his driving route out of Whitehaven and eventually to a wooded area in Boot where he turned the gun on himself.

He walked away from a family with two young children and the half-term holidays of that week meant he could have targeted children but he did not, the psychologist said.

"By that discrimination he knew what he was doing and he knew what he was doing was wrong," he said.

Forensic psychiatrist Dr Mark Swinton told the hearing that it was his opinion that Bird had a mental illness at the time of the incidents.

"In my view at the time he was suffering from a recognised mental illness - a mental illness characterised by delusional beliefs," he said.

"They were there three days before the incidents and there was a preceding period of five weeks before that where he was becoming mentally ill but where delusional beliefs were not obvious or apparent."

Asked by HM Coroner for North and West Cumbria, David Roberts, whether Bird could be classed as insane, Dr Swinton replied: "If he had lived I would have thought it extremely unlikely he would have been found not guilty of murder by reason of insanity.

"I am sure that a jury would have accepted he knew what he was doing and what he was doing was wrong."

The hearing continues tomorrow when the Assistant Chief Constable of West Mercia Police, Simon Chesterman, will give evidence about his yet unpublished report into Cumbria Constabulary's response to the shootings on the day.

Assistant Chief Constable of Cumbria Police, Jerry Graham, who was the Gold Commander for the incidents, will also go into the witness box.