Seoul (CNN) The only way to change North Korea's destiny is to change its leader, says the most senior North Korean diplomat to defect in almost 20 years.

Thae Yong Ho was number two in the North Korean embassy in London before he escaped with his wife and two sons, arriving in South Korea in August.

"As long as Kim Jong Un is in power, there'll be no chance for the world to improve the human rights issue" or cancel "the nuclear program," he says.

Thae's initial hopes that Kim's youth and overseas studies would make him a reformer were soon destroyed as he saw more and more of his fellow high-ranking officials being executed, almost, he says, on a whim.

"If Kim Jong Un decides to kill someone, if he thinks that he is a threat or he scared him, he just wants to get rid of him, that is the present reality of North Korea," he says, adding that he knows more elites will defect.

A South Korean think-tank affiliated with the country's intelligence agency (INSS) assesses at least 340 people have been ordered to be executed since Kim took power in December 2011.

Talking to Trump

Thae says after his initial surprise that President Donald Trump won the US election, Kim now sees it as "a good opportunity for him to open a kind of compromise with the new American administration."

But Thae insists Kim will only talk on his terms, pointing to the leader's New Year's Address.

"The level of this New Year's speech is almost blunt blackmail," says Thae.

Thae tells CNN's Paula Hancocks he was lucky to have his wife and children with him in London.

In the address, Kim made it clear that if the US continued its current policy against North Korea, he would continue to add nuclear weapons to the country's military capability.

"He called it pre-emptive strike capability," says Thae. "That is ICBM [intercontinental ballistic missiles]."

During his campaign, President Trump said he would be open to meeting Kim . Thae makes a plea for the president to reconsider, saying it would give the North Korean leader legitimacy he currently doesn't have in his own country.

"Even Chinese President Xi Jinping and even Russian President Putin -- they haven't even met Kim Jong Un," he says.

Thae claims that despite commanding loyalty through fear, Kim is still struggling to secure the legitimacy enjoyed by his father Kim Jong Il and grandfather Kim Il Sung, the founder of North Korea.

"After 5 years in power, he couldn't even tell the North Korean people about his date of birth, when he was born, about his mother, his relationship with his grandfather," he says.

Sanctions are working

Thae says sanctions passed last March by the United Nations Security Council -- described at the time as "ground-breaking" and "unprecedented" -- are working.

He says Kim wanted to establish 14 special economic zones inside North Korea and set up two ministries to encourage foreign investment, but these two ministries have since been dissolved.

He also says the sanctions are a "very, very strong factor" in the North Korean economy and have a "psychological effect on North Korean people and high ranking officials."

But they're not enough.

In keeping with President Trump's line, Thae says China needs to be persuaded to do more. He says it needs to prevent natural resources being smuggled across the border from North Korea for cash, and also has to take North Korea's nuclear program more seriously.

Thae is worried about the impact his actions will have on his relatives back in North Korea.

Thae even goes one step further -- saying that nuclear weapons aren't only a threat to the US and South Korea.

"According to international geopolitics there is no eternal enemy or friend... nobody can predict if one day Kim Jong Un may want to blackmail China," he says.

Cutting off the 'slavery chain'

When asked about relatives back in North Korea, Thae's voice drops.

He admits they may be sent to prison camps as punishment for his defection, or may even be used by the regime against him.

He knows he was extremely lucky to have his wife and both children with him in London, and that it's unlikely to happen to any other diplomat again.

He says North Korean diplomats will stay because their children are back in Pyongyang.

"The children are used by Kim Jong Un as kind of hostage," he says. "Kim Jong Un abuses even the love between parents and children."

When he finally told his wife and son of his intention to lead them all to safety, he says they were "very grateful."

He says he could not miss the opportunity to "cut off this slavery chain" for his sons, knowing they would never have forgiven him if, in the future, they knew he had not taken the chance for their freedom.

But defection is bittersweet for Thae Yong Ho.

"It made my life very miserable because I spent 50 years of my life on the wrong side, and I have to deny my past."