He was the intellectual force behind the philosophy of self-reliance that guided North Korea and a top official in the Workers' Party that still rules the communist nation.
He graduated from the elite university named for North Korean founder Kim Il Sung and was personally close to Kim himself. He tutored Kim's son, Kim Jong Il, who rose to become the peculiar, deified leader of the isolated state.
Then, in 1997, during a visit to China, Hwang Jang-yop sought asylum with South Korea _ triggering a five-week diplomatic standoff and earning him scorn from the regime back home in North Korea and the epithet "human scum" in its media.
Safely in South Korea, he spoke about the danger posed not just to the Korean peninsula but to the world by the dictatorship in the North, and said trying to persuade the North to give up its nuclear ambition was hopeless so long as Kim was in power.
Hwang's naked body was found Sunday morning in a bathtub at his home in Seoul, the South Korean capital, police said. Foul play was not initially suspected, but an autopsy was planned. Hwang was 87.
His death came as North Korea held a massive military parade to commemorate the 65th anniversary of the foundation of the Workers' Party. Kim Jong Il and his son, heir apparent Kim Jong Un, watched as armored trucks and tanks rolled by.
Since his defection, Hwang had lived in Seoul under tight police security amid fears that North Korean agents might try to take revenge. He wrote books and delivered speeches condemning Kim's government as authoritarian.
Two North Korean army majors were sentenced to prison in South Korea in July for plotting to assassinate him. North Korea has denied the plot, accusing South Korea of staging it to intensify sentiment against the North.
In February 1997, Hwang was party secretary for international affairs and ranked 24th in the hierarchy of the reclusive North. He had attended an international seminar in Japan and had stopped in Beijing on his way back.
When he and an aide sought asylum with the South Korean diplomatic mission, it put China in an awkward position, caught between the dictatorship in Pyongyang, its traditional ally, and its growing trading partner to the south.
China subsequently asked the Philippines to allow Hwang to travel there first rather than directly going to Seoul in an effort not to anger North Korea. Two South Korean fighter jets escorted Hwang's plane flying in from Manila, 67 days after he defected.
The North at first accused the South of kidnapping Hwang and threatened unspecified retaliation, and South Korea put its army on high alert. But the North later said that it had decided to banish Hwang, calling him a betrayer.
South Koreans hailed the defection as an intelligence bonanza and one of the clearest signs that North Korea's half-century experiment with communism had failed and its political and economic systems were inferior to their own.
Kim Jong Il, meanwhile, reportedly vowed revenge for the defection of his former ally. The two North Korean agents convicted of plotting his assassination were each given 10 years in prison.
They posed as ordinary defectors and told investigators they were ordered to report back to Pyongyang on Hwang's activities in Seoul and to prepare to "slit the betrayer's throat," according to South Korean prosecutors.
High-profile defectors from North Korea are believed to have been targeted for assassination. Also in 1997, a nephew of one of Kim Jong Il's former wives was killed outside a Seoul apartment 15 years after fleeing to the South. Officials never caught the assailants but believed they were North Korean agents.
Hwang was skeptical about international efforts to persuade North Korea to give up its nuclear programs as long as Kim Jong Il remained in power.
"It is nonsense to urge the North to abandon its nuclear weapons with Kim in place," he told The Associated Press in a 2006 interview in Seoul just days after the North had carried out its first underground nuclear test.
Hwang rarely traveled abroad because of the concerns for his security. He did visit the United States in 2003 and in March of this year, and also went to Japan in April. He used the opportunities to criticize the North.
"The dictatorship of North Korea is not a problem that is limited to the Korean peninsula, but ... a problem that all the people in the world must deal with," he told AP in an interview in Washington in 2003.
The Network for North Korean Democracy and Human Rights, a Seoul-based civic group concerned with North Korean affairs, expressed condolences over Hwang's death, noting he was a leading campaigner to bring democracy to the North.
"We express sympathy and condolences to Hwang's family and relatives who could not even know" of his death, the civic group said, referring to those he left behind in North Korea.
He had a wife, two sons and a daughter in North Korea before his defection.
South Korea's ruling Grand National Party said in a statement that it prayed for Hwang's soul, saying he dedicated himself to "getting people to know the actual circumstances of North Korea and to improving North Koreans' human rights."
Kim Young-sam, who was president of South Korea at the time of the defection, also mourned Hwang's death, praising him as a "great patriot" who sharply criticized the North's dictatorship, according to Yonhap news agency.