Kim was an outspoken critic of Koreans who collaborated with the Japanese during occupation and often protested against the government police force on Jeju, demanding unification with the North.

Kim was vehemently opposed to the elections planned for 10 May 1948 by the United Nations Temporary Commission on Korea (UNTCOK) because he thought they would further reinforce Korean division.

[1]: 156–157  In order to prevent these elections from happening, Kim led an armed rebellion against the government police forces on the island on 3 April 1948, which became known as the Jeju uprising.

[1]: 159 As a leader in the Military Committee of the WPSK Jeju branch, Kim, along with Cho No-gu, planned the 3 April 1948 attacks on government police forces one month prior, in March 1948.

[1]: 174 [2]: 31 In August 1948, with the uprising in full-swing, Kim left Jeju to attend a conference of the Korean Communist party (KCP) on the border town of Haeju, North Korea.