I have always thought that Christianity is the biggest threat to the North Korean government because it directly challenges the regime’s cult of personality based around the Kim family.  Judging by how passionately and widespread Christianity has been embraced by South Koreans obviously the regime is taking the threat of Christianity very seriously be executing those who preach it:

Like most North Koreans, Son Jong Nam knew next to nothing about Christianity when he fled to neighboring China in 1998.

Eleven years later, he died back in North Korea in prison, reportedly tortured to death for trying to spread the Gospel in his native land, armed with 20 bibles and 10 cassette tapes of hymns. He was 50.

His story, pieced together by his younger brother, a defector who lives in South Korea, sheds light on a little-discussed practice: the sending back of North Korean converts to evangelize in their home country – a risky move, but one of the few ways to penetrate a country that bars most citizens from outside TV or radio and the Internet.

Little is known about the practice, believed to have started in the late 1990s. Missionaries won’t say how many defectors they have sent back, citing their safety and that of the defectors.

“It’s their country, where people speak the same language. They know where to go and where to escape,” says the Rev. Isaac Lee, a Korean-American missionary in Seoul who has dedicated his life to spreading Christianity in the North. “But I agonize a lot whenever I have to send defectors to the North as I know what kind of punishment they would get if arrested.”

Officially, North Korea guarantees freedom of religion for its 24 million people. In practice, authorities crack down on Christians, who are seen as a Western-influenced threat to the government. The distribution of bibles and secret prayer services can mean banishment to a labor camp or execution, defectors say.  [Stars & Stripes]

You can read the rest of this article at the link, but there is no doubt that the South Korean Christian groups working in China have done far more for the people of North Korea than the so called international human rights groups who care more about promoting themselves than promoting North Korean human rights.