The demand for the death penalty for a former South Korean president for his abortive attempt at imposing martial law more than a year ago deepens a right-left political divide in South Korea that threatens Seoul’s ability to stand up against rising threats from North Korea.

The prosecutor’s decision to seek the execution of the conservative Yoon Suk-yeol, impeached and jailed, has come as a shock to Koreans who predict rightists will make him a hero in their battle against Mr. Yoon’s successor, the left-leaning Lee Jae-myung, elected president in June after a court approved Mr. Yoon’s ouster.

“We are totally divided,” a young Korean office worker, Chang Yong-jin, tells the Sun. “Nobody thinks Yoon will be executed. People will rally around him, but the country will suffer.”

North Korea’s leader Kim Jong-un “will be the only one to benefit from this case,” says a conservative politician, Choi Tae-Hyun, contacted by the Sun, “Mr. Yoon is innocent. North Korea builds its relationship with Russia, and we are in danger of civil war.”

The ordeal of Mr. Yoon and the generals and other officials who supported him in his attempt at repressing his foes, who hold a majority in the National Assembly, could also strengthen Mr. Lee’s efforts at drawing North Korea’s leader, Kim Jong-un, to resume dialogue with the South. Mr. Kim “will exploit the case,” says Mr. Choe. “Their agents in the South are secretly supporting far leftists”

Mr. Yoon, talking for 90 minutes after the prosecutor recommended the death penalty, said his foes had “sought “excessive indictments” against “countless public officials who faithfully performed their duties.” The claim that he had “participated in an insurrection,” he said, was “mere delusion and fiction.”

While likely to be found guilty, Mr. Yoon is much more likely to be sentenced to life in prison rather than death. Eventually the death penalty could be commuted, or he could be freed by a new president.

That’s what happened to the one late president, Chun Doo-hwan, given the death penalty for taking over the government after the assassination of the long-ruling Park Chung-hee in October 1979 and then ordering soldiers to fire on civilians, killing nearly 200 in the southwestern city of Gwangju in May 1980. The intelligence chief who assassinated Park was hanged while Chun held power for nearly eight years before being tried himself, sentenced, and finally freed in the interests of national unity.