It's not particularly wild because the South Korean government was almost entirely fascist collaborators who had run the colonial government for Japan, and who were reinstalled by mass violence and the crushing of dissent, with US approval. The SK government killed several hundred thousand of their own citizens, see the pictures of the Bodo League massacre taken by American officers for example.

[1]

> Estimates of the death toll vary. Historians and experts on the Korean War estimate that the full total ranges from at least 60,000–110,000 (Kim Dong-choon, who stated that this was likely a very conservative estimate) to 200,000 (Park Myung-lim).

> The massacre was committed by the government forces of president Syngman Rhee and falsely blamed on the communists led by North Korean leader Kim Il Sung. The South Korean government made efforts to conceal the massacre for four decades. Survivors were forbidden by the government from revealing it, under threat of being treated as communist sympathizers; public revelation carried with it the threat of torture and death.

Moreover, the Chinese did not enter the war until UN forces had driven all the way to the Manchurian border, along with regular "accidental" cross-border air attacks. MacArthur, Kai-shek, and Rhee all fully desired war with China and were hoping to escalate the conflict.

I've posted these quotes before, copied below:

[2]

> In the fall of 1946, the US military authorized elections to an interim legislature for southern Korea, but the results were clearly fraudulent. Even General Hodge privately wrote that right-wing "strong-arm" methods had been used to control the vote. The winners were almost all rightists, including Rhee supporters, even though a survey by the American military government that summer had found that 70 percent of 8,453 southern Koreans polled said they supported socialism, 7 percent communism, and only 14 percent capitalism. [...]

> Chung Koo-Hun, the observant young student of the late 1940s, said of the villagers' attitude: "The Americans simply re-employed the pro-Japanese Koreans whom the people hated." [...]

> Seventy of the 115 top Korean officials in the Seoul administration in 1947 had held office during the Japanese occupation.

> In the southern city of Taegu, people verged on starvation. When 10,000 demonstrators rallied on October 1, 1946, police opened fire, killing many. Vengeful crowds then seized and killed policeman, and the US military declared martial law. The violence spread across the provinces, peasants murdering government officials, landlords, and especially police, detested as holdovers from Japanese days. American troops joined the police in suppressing the uprisings. Together they killed uncounted hundreds of Koreans.

> American anthropologist Cornelius Osgood, spending much of 1947 in a village west of Seoul, watched as police carried young men off to jail by the truckload. A "mantle of fear" had fallen over once peaceful valleys, he wrote. The word "communist," he said, "seemed to mean 'just any young man of a village.'" On August 7, 1947, the US military government outlawed the southern communists, the Korean Worker's Party. Denied a peaceful political route, more and more leftist militants chose an armed struggle for power.

[1] - https://en.wikipedia.org/wiki/Bodo_League_massacre

[2] - quotes from The Bridge at No Gun Ri

[3] - https://en.wikipedia.org/wiki/Jeju_uprising

[4] - https://en.wikipedia.org/wiki/Gwangju_Uprising (note that this was in 1980, under a different dictator that the US also backed)