(CNN) North Korea's former deputy ambassador to the UK, who defected to South Korea in 2016, tells CNN he was like "a modern slave."

"I did not want to let my sons lead a life like me," Thae Yong-Ho told CNN's Christiane Amanpour on Thursday from Washington, where he was testifying before Congress.

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Thae, as a senior diplomat, was exposed over many years to a grim juxtaposition: The draconian politburo that controls his country, and the freedoms of the countries in which he served.

That was only heightened in recent years, under the leadership of Kim Jong Un.

"He faced some challenges inside the leadership of North Korea," Thae explained. "That's why Kim Jong Un decided to show he is a man with mercilessness -- so that everyone should be frightened about him."

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