It's nerve-racking to drive toward the North Korean border with Park Sang Hak. Called "Fireball" by his admirers, the North Korean-born Mr. Park is designated "Enemy Zero" by the Pyongyang regime, which two years ago sent an agent into South Korea to assassinate him with a poison-tipped pen. On this summer morning, he promises to do again what so infuriates the Kim dictatorship launch large balloons into North Korea carrying leaflets, computer-memory sticks and sweets for the oppressed people of the hermit kingdom.

In return, Pyongyang promises to "physically eliminate the kind of human scum that commits such treason." Adds the North Korean military: "The U.S. and the present puppet authorities of South Korea should not forget even a moment that the Rimjin Pavilion" Mr. Park's favorite launch site near the Demilitarized Zone separating the two countries�"is within the range of direct sighting strike" of the Korean People's Army.

North Korean threats are generally discounted as bluster, but driving toward the border has a way of concentrating the mind. The highway from the South carries little traffic in either direction and is separated from the Imjin River by barbed-wire fences, guard towers and civil-defense loudspeakers.

Yet once we arrive at the balloon-launch site, it becomes clear that Mr. Park has a different antagonist to contend with: South Korea, which has deployed a few hundred police to stop his airborne humanitarian mission. The police let him speak to assembled local media, but when Mr. Park tries to retrieve his balloons from a pickup truck, three rows of plainclothes officers block the way. Mr. Park tries to push through, but the police push back. When he tries to drive the truck slowly through the cordon to a different launch site, a large scuffle breaks out.

Shoving matches pit uniformed and plainclothes police against Mr. Park and fellow activists most of whom, like him, are native North Koreans who defected to the South sometime in the past 15 years. The most recent defectors wear handkerchiefs to cover their faces, because their identification in photos could mean imprisonment or execution for family members left behind. The grim, chaotic scene ends when Mr. Park is shoved into a police car and driven to the Paju Police Station.

He is released later that day, but not without a reinforced sense that something is rotten in one of the world's most prosperous democracies. "Launching balloons is legal activism that I can do as a free citizen," he tells me (through an interpreter) when we meet again two days later. But his arrest shows that "the threatening and blackmailing by the North Korean regime work well in the international community." And especially in Seoul, he argues, where successive South Korean governments have played down the catastrophic human-rights abuses across the border.

Mr. Park is one of roughly 25,000 North Koreans to escape and get to the South, and his balloon launches aim to break the information monopoly held by the Kim family. In addition to praising the South's liberal democracy, the leaflets reveal unflattering truths about Pyongyang's rulers, such as Kim Il Sung's craving for mistresses during his reign from 1948-94. The digital memory sticks inform North Koreans about the world, including "people power" movements and life inside South Korea as captured by news articles, movies and (especially) soap operas.

The idea, says Mr. Park, is that even in North Korea "the truth can set you free"but only if you have access to it. "We don't want the South Korean government or the U.S. government to start a war," he says. "What we're waiting for is to change the [Pyongyang] regime by the hands of North Koreans who are educated with the truth. . . . That's the only way we can give freedom to the 24 million people in North Korea."

This strategy of pursuing regime change from within was explicitly rejected by South Korean leaders during the "Sunshine Policy" years of 1998-2008. Seoul believed that Kim Il Sung's successor, his son Kim Jong Il, would lose interest in nuclear weapons and loosen his grip at home if he saw that no outside forces were working to oust his regime. So South Korean presidents feted him at summits, gave him economic benefits, and ignored human rights.

"Sunshine" didn't stop North Korea's nuclear drive, and South Korean voters have since rejected it twice at the polls. But five years of conservative leadership in Seoul haven't made life easier for Mr. Park, who can't recall whether his latest arrest is number seven or eight.

"South Koreans like saying that we are all in the same family, same ethnic group, we share the culture but they're just saying that, not really feeling it or believing it," he says. "Many South Korean people think that when the Koreas are unified, they will have to take all the economic burden for developing North Korea, which is deprived and underdeveloped, so will have to pay more taxes for that."

Mr. Park has little sympathy for this dollars-and-cents concern, even as he acknowledges South Korea's efforts to resettle 25,000 defectors, including him and his family. "The GDP of South Korea is $23,000 per person," he notes. "After reunification, after the North Korean regime collapses, the North Korean people will come and see this affluence and they're going to ask you, 'What did you do when we were suffering back in North Korea?' What kind of answer should we give to them?"

The problem is visible, says Mr. Park, in how South Korean journalists report on North Korea's new dictator. "They mention Kim Jong Un even more than North Korean media," he says, sounding incredulous and adding that reporters in the South speak of Pyongyang as if it were a normal government, not a totalitarian tyranny. "They call Kim Jong Un by his formal names. . . . It's like calling Hitler by his full rank and title, to pay respect."

Park Sang Hak was born in 1968 into the elite of North Korean society. His father was a senior official in the Workers' Party with responsibility for smuggling computer technology into North Korea from Japan and elsewhere. The work earned Mr. Park's father riches, large amounts of U.S. currency, an Omega watch from "Great Leader" Kim Il Sung, even a Mercedes-Benz and eventually a promotion into the intelligence directorate responsible for sending spies into South Korea. But he decided to defect in 1999, while stationed in Tokyo, telling his family that with North Koreans starving to death, to aid the Kim regime was a crime against the people.

The younger Mr. Park didn't want to leave, since at age 31 he was a rising bureaucrat on the make. "I knew that within a few years I was going to be a member of the Party, which is really prestigious. I didn't even serve in the military which is allowed only for elites but I still got the membership." He owned a car, an imported Toyota. "I blamed my father for abandoning his faith in communism and becoming a capitalist," he recalls.

But over a two-month period, as he listened to a taped message sent secretly by his father, young Mr. Park began to see things differently. "I felt for the first time that I wasn't working for people but rather reigning over people," he says. "I felt like a criminal. I felt guilty for spending a lot of money and having a luxurious life based on the efforts and miserable lives of ordinary citizens."

On Aug. 9, 1999, he, his mother and two siblings defected across the Yalu River into China, having pleased the border guards with 10 times the usual bribe.

Once in South Korea, Mr. Park didn't immediately take up the anti-Pyongyang cause, working instead at Seoul National University. His spur to activism came in 2003, when he learned that as a result of his family's defection, the regime had tortured his uncles to death. His cousins had become street beggars, and to this day he doesn't know if they are alive. "After I learned of that," he says, "I had to stand up and do something."

Still, he has some gripes with the U.S., especially the Bush administration's 2008 decision to remove North Korea from the list of state sponsors of terrorism. But Mr. Park doesn't fault Americans for thinking of North Korea more as a nuclear threat than a human-rights violator. For that he blames South Korea: "If your house is on fire, and you don't care much about it and run away, you can't expect other people to extinguish it for you."

Mr. Park notes that South Korea's president visited China this week without publicly pressing Beijing to stop repatriating defectors to North Korea, where they face imprisonment or worse. In Washington, meanwhile, North Korea remains off the terror-sponsor list, and U.S. officials want to resume negotiations with the Kim regime. "They're going to deceive you again and again and again," Mr. Park warns.

At least he offers some consolation as our conversation ends: Most of his balloon launches take place in secret without advance notice or the glare of
cameras so the South Korean authorities let them proceed. Thus 30 to 40 times a year, when wind conditions are favorable and donors provide sufficient funds, Mr. Park launches 200,000 leaflets northward toward the hermit kingdom. Each one is considered a threat by Pyongyang. No regime so fragile can last forever.

Mr. Feith is a Journal editorial page writer based in Hong Kong.
This article is from "The Wall Street Journal" July 5, 2013