Thae Yong Ho defected to South Korea in August last year and has been openly talking to the media about his concerns over the country’s direction under the young leader.
At the time he said he had fled the regime because he was disgusted with his homeland for repressing its people.
His defection was all the more surprising since he had addressed rallies and gatherings in London to defend his country’s foreign policies and vehemently rejected western media’s analysis of Kim.
He told a news conference on Wednesday: “When Kim Jong-Un first came to power, I was hopeful that he would make reasonable and rational decisions to save North Korea from poverty, but I soon fell into despair watching him purging officials for no proper reasons.”
He went on: “Low-level dissent or criticism of the regime, until recently unthinkable, is becoming more frequent.
“We have to spray gasoline on North Korea, and let the North Korean people set fire to it.”
Mr Thae, 54, has said publicly that dissatisfaction with Kim Jong-Un prompted him to flee his post.
His two university-age sons living with him and his wife in London also defected with him.
North and South Korea are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty.
The North, which is currently being punished with United Nations sanctions over its nuclear and missile programmes, regularly threatens to destroy the South and its main ally, the United States.
Mr Thae is the most senior official to have fled North Korea and entered public life in the South since the 1997 defection of Hwang Jang Yop, the brains behind the North’s governing ideology, “Juche”, which combines Marxism and extreme nationalism.
He said today’s North Korean system had “nothing to do with true communism” and added that the elite, like himself, had watched with unease as countries such as Cambodia, Vietnam and the former Soviet Union embraced economic and social reform.
Mr Thae has said that more North Korean diplomats are waiting in Europe to defect to South Korea.
Asked if Kim Jong-Un’s brother, Kim Jong-Chol, could run the country instead, he remained sceptical.
He said: “Kim Jong-Chol has no interest in politics. He is only interested in music.
“He’s only interested in Eric Clapton. If he was a normal man, I’m sure he’d be a very good professional guitarist.”