It's not over yet ... Why did Japan sink the Ukishima Maru?

Billed as a North Korean "Titanic". On August 15, 1945, Japanese Imperialists were defeated and there echoed in Japan, too, shouts of victory of thousands of Korean people who had been drafted to Japan. They formed a repatriation autonomy to return to their dear homeland. The Japs plot to blow up their ship on the way which is overloaded with thousands of Koreans who requested repatriation; whose number in fact is twice as many as the ship's fixed capacity. Cha Myong Jin and other Koreans who are ignorant of this fact set out with great joy and hope, talking and laughing. Cha Myong Jin meets his fiance on the ship and whispers their future life. But they dream breaks out. The Japs turn the ship to the Maizuru Port under the pretext of refilling the drinking water and drive the Koreans below deck while preparing to blow up the ship.