When the Crown Prince of Nepal shot dead nearly his entire family last June, one journalist described it to me as the first genuine news event she could remember. September 11 was yet to come. Since I had written about India, and with India being close to Nepal, it was suggested that I might go out and write about it. I was undecided, but the late lamented Talk magazine was offering lots of space and even higher rates than the Literary Review, so I said yes.

In Kathmandu, demonstrations were being broken up, local journalists were being arrested, cars were burning on the streets and most Nepalese were in a state of hysterical shock. The actions of Crown Prince Dipendra were so unbelievable that many people preferred not to believe them, and instead concocted wild conspiracy theories about what had gone on inside the royal palace. It was the CIA; it was his uncle; there were accomplices; Dipendra had been cloned, and the clones had done the killing while the real Dipendra was abducted. Mohamed Al Fayed may even have thought the Duke of Edinburgh had had a hand in it.

Nepal’s official press led with headlines such as ‘Their Late Majesties Mourned’ and ‘President of Latvia Pays Tribute’, while the rest of the world piled in with tales of gore and thwarted love.

It was the first time I had seen the international media out in force, and it was an impressive and unnerving sight. It was bewildering how much equipment television journalists needed, and I wondered how they got anyone to speak on camera when most people were nervous of speaking at all. It was also a lesson in national approaches to reporting. The Japanese focused on detail; the British were interested only in sensation; Americans checked facts but sometimes missed the point; Indians invented stories and then presumed they were true because they had read them in a newspaper. Strangest of all, many journalists seemed to regard other journalists – and white expatriates living in Kathmandu – as the most reliable source of information.

After a week or so, the circus departed, and it was only me, Vanity Fair and the New Yorker left, circling each other in the lobby of the Yak and Yeti Hotel. Events in the palace on the night of the massacre had by now been established by an official inquiry: in Cluedo terms, it was the Crown Prince, acting alone, in the billiard room, with a Heckler & Koch MPSK machine gun. Dressed in combat fatigues, he had murdered his father, mother, sister, brother and assorted relations during a family party, before shooting himself in the head with a pistol. His parents’ refusal to let him marry his girlfriend, Devyani Rana, appeared to have been the proximate cause of the killing.

My task was to find out what had escalated a conventional family dispute into mass murder. Talk magazine rang up every hour or so, asking for the latest update; I would switch on CNN, and tell them. Never having written an investigative report before, I was unsure where to start, and spent a lot of time in the hotel swimming pool, thinking. It seemed that the Prince’s bodyguards, and royals of his own age, would be likely to know what had really been going on. I went in search of them, and after several false leads the story became clear.

Dipendra’s disturbed mental state and obsession with guns were at the root of what had happened. In early 2000, he had secretly been to see a psychiatrist, who put him on antidepressants and told him to cut down on his drinking. This worked for a while, but he stopped taking the pills when the symptoms disappeared. He then visited a second psychiatrist, but ignored the advice he was given. When he assaulted his pregnant sister a few months later, and threatened to kill his whole family, his aunt suggested to the Queen that he be sent abroad for treatment. She refused, saying that crown princes did not need therapy. Since he had taken to keeping automatic weapons in his bedroom, and was smoking large amounts of dope, this emphasis on protocol was unwise. The Queen would lose her life, and many others would have their lives destroyed. I remember the terrible grief of Dipendra’s cousins, whose entire extended family had been wiped out, as they described the effect of the massacre on their own children. One little boy went to bed each night with his hands over his eyes, saying, ‘When I cover my eyes, I can still see my granny.’

Jonathan Gregson has written a clear, professional account of these events. The first half of his short book covers the history of Nepal’s royal dynasty, with a panoply of plots, scheming, torture and bloodletting. He compares past rulers to Frederick the Great and Napoleon, but avoids the central fact that Nepal has always been of marginal economic and political significance. In his version of the events leading up to Dipendra’s rampage, he sticks to the known facts, and avoids stories and rumours that cannot be substantiated. He shows the dysfunction of the royal family, and the limitations of the role of a crown prince: ‘The only ways out are to abdicate, be declared insane, or commit suicide.’ In his view, the motive for the massacre was consumed in the ashes of the dead crown prince’s funeral pyre’. This is not quite true; the person who could elucidate it, Dipendra’s girlfriend Devyani, has refused to speak about that fatal night.

After leaving Nepal, I had a chance conversation with a recovering drug addict, who had spent a lot of time in Kathmandu . Discussing Dipendra’s motivation, he said, by way of explanation, ‘If you spend long enough at the barber’s, you get a haircut.’ At the time I dismissed this as a Withnail and I-ish remark , but, looking back, I think that he was probably right. If you spend long enough in a state of paranoid anger – stoned, drunk, nursing grudges against your parents , watching violent movies, with a rack of sub-machine guns in your bedroom, all the while being treated like royalty – then sooner or later you will explode and take action. Dipendra did it because he could.