He was a key member of North Korea's first family, a man widely seen as regent to leader Kim Jong-un, but in a dramatic twist worthy of an episode of crime series The Sopranos, the instigators of Jang Song Thaek's execution may have included his wife.

The final decision to execute Jang, last seen publicly being frog-marched by armed guards from a special party session last week, was probably made by Kim, his nephew, and Kim Kyong-hui, his wife, according to several sources.

News of Jang's execution was accompanied by a string of extraordinary insults, branding him a "traitor for all ages" and "despicable human scum" who was "worse than a dog." A 2,700-word state media report of his trial in a special military tribunal on Thursday said he had admitted to plotting insurrection and a string of other crimes.

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"He let the decadent capitalist lifestyle find its way to our society by distributing all sorts of pornographic pictures among his confidants since 2009." The report said Jang led a "dissolute, depraved life" and had squandered at least 4.6 million euro from state coffers on gambling. He was executed, probably by firing squad, immediately after the tribunal.

Jang's killing is the highest-level purge since Kim Jong-un inherited power from his father Kim Jong-il in 2011 and has left opinion divided on what it means. Many experts say Kim had no choice but to remove his powerful but corrupt uncle if he wanted to graduate from young pretender to dictator "He had to go," says veteran Pyongyang watcher Andrei Lankov. "To really start running the country Kim must get rid of the old guard. They are so much older; they are in their sixties and seventies and he is in his thirties."

But even if Jang's removal was operationally logical, the violence of his public humiliation and disposal was highly unusual, accepts Lankov. "One possibility is that he wanted to terrify everyone, to show that he is young but someone to be afraid of, to show that nobody is immune," he says. "It might also reflect his personal animosity to Jang. He did not like the man, who probably bossed him around."

South Korea fears Jang's ouster could trigger political turmoil and a wave of defections by some of the thousands of loyal cronies he brought onside since marrying the daughter of state founder Kim il-Song in 1972. The Chosun Ilbo newspaper says US and Chinese spy agencies are "racing to recruit" a senior confident of Jang's who has already fled the Pyongyang.

Jang was intermittently at the center of power in North Korea. He is widely thought to have been purged by his brother-in-law, Kim Jong-il from 2004-6 - punishment for flaunting his opulent cadre's lifestyle. As Kim's health ebbed before his death in 2011, he began leaning on trusted family members - his sister, son and brother-in-law - during the transfer of hereditary power to his 28-year-old son.

But Jang was always handicapped by his lack of blood ties to the first family, says the North Korea Strategic Information Service Centre, an organisation run by elite defectors from the North's government. "The key to succession" in the North is the Kim bloodline, said Lee Yun-keol, head of the centre. Jang's execution sends a "chilling message" that it's leadership cannot tolerate challenges to the bloodline. "The final decision of Jang Song Thaek's ouster was made by Kim Jong-un and Jang's wife Kim Kyung-hui."

As with most of the North's elite, information on Jang is sketchy. He was for years head of the country's internal security, an elite enforcer who locked up enemies of the state. He was widely seen by analysts as corrupt and bribable, largely without strong political convictions. "Precisely as charged, he was a womanizer and substance abuser, accustomed to being wined and dined," said Bradley K. Martin author of Under the Loving Care of the Fatherly Leader, on the Global Post website.

State news agency KCNA claimed that Jang admitted plotting to stage a coup using "high-ranking army officers" and other close allies. "I didn't fix the definite time for the coup," he reportedly said. "But it was my intention to concentrate my department and all economic organs on the cabinet and become premier when the economy goes totally bankrupt and the state is on the verge of collapse."

The ruthlessness of his purge has taken many analysts by surprise. "With Jang Song Thaek gone, there's nobody else to execute," said Victor Cha, a former senior White House advisor on North Korean affairs, on the online news site NKnews.org. "When you take out the key elements of the party and the key elements of the military you're kind of building from scratch again. It's a very risky strategy."

Jang's wife, who is reportedly ill, has made no public comment about his death, leaving the narrative of her husband's execution - and its aftermath - entirely in the hands of state propagandists. The KCNA said the country embraced the news. "The DPRK army and people are now advancing toward the rosy future of a thriving socialist nation, single-heartedly rallied around Kim Jong-un," said a statement. "In this new era … there is no room for a handful of political careerists and factionists to live in."