SEOUL — The blind cruelty of one of South Korea’s former leftist presidents and the rash arrogance of its recently impeached conservative president have been on display in separate courtrooms here.

For cruelty, the decision of the South’s earlier president, Moon Jae-in, to send two North Korean fishermen back to their country after the North claimed they had murdered their 16 mates was evident in the verdict handed out to four one-time top officials in Mr. Moon’s government. More than five years after the two fishermen defected to the South in their boat, the four got prison sentences ranging between six and 10 months for surrendering them to the North Koreans.

A district court acknowledged that the decision to deport the fishermen “was made quickly based on their confessions” — after they were denied legal counsel or the chance to challenge their “confessions,” widely viewed as forced or bogus. Yet the ex-officials won’t go to jail, as the court suspended their sentences, meaning they’ll eventually be exonerated.

The district court implied that the case against the four ex-officials was motivated by the conservative government of President Yoon, Mr. Moon’s successor. Mr. Yoon was impeached by the national assembly in December for declaring martial law. The country’s constitutional court is almost finished reviewing his impeachment, after which the court is to decide whether to approve it and oust him as president. He still faces charges of “insurrection” in a lower court for his martial law decree, which Korea’s Democratic Party, or Minju — to which Mr. Moon belongs — voted down.

For arrogance, the refusal of Mr. Yoon to heed the advice of his prime minister, Han Duck-soo, not to impose martial law was spotlighted as the constitutional court weighed whether to approve his impeachment. Automatically named acting president after Mr. Yoon’s impeachment, Mr. Han lost that post when the assembly impeached him for balking at naming three justices to fill vacancies on the nine-member court. Needing six to oust the president, that court now has eight sitting members.

Mr. Han testified that he had “failed to convince” Mr. Yoon not to impose martial law. “I did not have prior knowledge of the president’s plans and did my best to persuade the president to reconsider,” he insisted. Mr. Han said he “had no part” in ordering troops to surround the assembly before the Minju rounded up the 200 votes needed for the assembly’s 300 members to impeach Mr. Yoon.

The cases of Mr. Yoon and the two North Korean fishermen, though very different, epitomize the deepening left/right fissure in Korean politics and society.

The four ex-officials who conspired to send the two fishermen back to sure torture and death in North Korea at the behest of Mr. Moon were serving as his chief of staff,  security adviser, intelligence director, and unification minister.

Mr. Moon at the time still dreamed of achieving North-South Korean reconciliation. Yet after President Trump walked out of his second summit with the North Korean leader, Kim Jong-un, at Hanoi in February 2019, Mr. Kim had no time for Mr. Moon. While Mr. Trump professed to have fallen “in love” with Mr. Kim at their first summit, in Singapore, the North Korean steadfastly refused to give up his nukes. 

Mr. Moon and his aides blindly accepted North Korea’s claim that the two fishermen had axed 16 others on the same boat. That claim had to be false, as they could not possibly have killed all 16 muscular men on the boat and then escaped by sea with no signs of a fight.

The quick court case was not uncovered until a photographer happened to see an email. Five days after the fishermen defected, they were photographed being transferred into North Korean custody at the truce village of Panmunjom. One of them was seen kicking and screaming as South Korean guards handed them over.

“The case against them was nothing but a fake,” the leader of mass protests against Mr. Yoon’s impeachment, Reverend Jeon Kwan-hoon, said. He accused the constitutional court’s chief justice of ties to the Minju, while several thousand demonstrators waved Korean and American flags, hefting signs saying, “Stop the Steal” and “Make Korea Great Again.”