Paras Shah has been unpopular among the Nepali people
Nepal's former crown prince has spoken about the 2001 royal massacre in which his cousin is said to have killed nine family members before killing himself.
The former heir to the throne, Paras Shah, said his cousin Prince Dipendra was angry at a failed arms deal.
Prince Dipendra was also unhappy at his family's rejection of his choice of bride, he said.
An official inquiry blamed Prince Dipendra for the royal massacre which shocked Nepal and the world.
Mr Shah is the son of Nepal's last king, Gyanendra Shah. The monarchy was abolished in the country last year.
The former crown prince left the country last July for Singapore where he lives now.
Speaking to the Singapore-based New Paper, Mr Shah said Prince Dipendra had not one but three reasons for wanting to kill his own father.
"The Nepali army was looking for a new weapon to replace the Belgian SLR. Dipendra liked the German Heckler & Koch G36 assault rifle, as opposed to the battle-tested Colt M16," Mr Shah was quoted by the newspaper as saying.
Prince Dipendra had 'three reasons to kill his father'
"But his father, His Majesty [King Birendra], did not agree. I know that they argued over it. Dipendra was frustrated. He wasn't happy. He told me," he said.
Mr Shah said the deal would have given Dipendra a windfall of about $15m.
The BBC's Charles Haviland in Kathmandu says it is an allegation which - if true - would confirm accounts of the former royals' corruption.
Mr Shah said a second motive for Prince Dipendra was his unhappiness at the decision of King Birendra to end absolute monarchy in Nepal.
"Dipendra was never the same after his father told him in 1990 about the plans to give up the monarchy.
"He never agreed with that as he wanted to rule the country. I think he started planning his moves then," the newspaper quoted him as saying.
Mr Shah said Dipendra was discontented for a third reason, namely, his family's veto on his wish to marry Devyani Rana.
"The royal family did not want Prince Dipendra to marry her as she was from a rival family," he said.
Soon after the royal massacre, an official investigation found that a drunken Crown Prince Dipendra had killed his parents, the King and Queen of Nepal, and seven other royals before committing suicide.
Mr Shah dismissed the notion that Prince Dipendra was drunk or that he had shot his family on impulse.
"I know him and I know when he had had too much to drink. He said he had been drinking since the afternoon but there was no smell of alcohol on him.
"How can that be? If he had been drinking all the while, he should have been reeking. But there was no smell," he said.
Mr Shah said Dipendra had talked to other young royals about his plan to "bring down the ivory tower" of the monarchy a full year before it happened.
But, he said, "how could we take him seriously"?
Mr Shah said he did not want to attend the party at which the killings happened, but that Prince Dipendra insisted he should be there.
The killings shocked ordinary Nepalese, many of whom refused to believe that Dipendra was responsible.
Many Nepalis accused King Gyanendra and his son Paras of plotting the massacre - even though the official inquiry and eyewitnesses said the killer was Dipendra.
Paras Shah was the first in line to the throne and was unpopular among Nepalis for his drunken antics and playboy lifestyle.
Our correspondent says his interview appears to have been an attempt to affirm his innocence, given the new government's intention to reinvestigate the atrocity.