Good question. In a recent book about North Korea, called The Cleanest Race, the author (an historian) puts forth the argument that the totalitarian dictatorship of the Kims was a continuation of the totalitarian dictatorship of the Japanese. That is, it was easier to subjugate a people who had never known freedom in living memory (almost), since they'd been ruled since 1905 by a dictator. It's a persuasive argument. But I'm simplifying. The main reason for the oppression of North Korea is the Kims and Soviet pressure, not historical oppression.

The Korean Nationalism was just building up and there were grassroot efforts from Koreans, exiled or not, to free themselves from Japanese Empire and regain independence. The problem is that there existed several, disconnected independence movements. According to Source of Korean Tradition Vol II, there existed two branches of independence movements: Nationalistic and Communistic. Although there were efforts to unite these movements into one, Japanese Empire purposely put them against each other and inhibited reconciliation between Nationalists and Communists. Division of North and South is result of the legacy of Japanese Empire.

After Japanese Empire was forced out, Koreans formed their own proto-government called "Korean People's Republic," which were group of influential independence fighters and thinkers who aimed to replace the Japanese government (This is mentioned in Source of Korean Tradition and Under the Black Umbrella). However, US and USSR dismissed the group saying how Koreans had no ability governing themselves and set up their own governments: one controlled by Kim Il Sung, which had been handpicked by Soviet themselves, and one controlled by former collaborators of Japanese Empire, who knew how to run a modern state, but considered as traitors from Koreans themselves. Although Koreans attempted to reconcile Nationalist and Communist factions, this failed in the end and Korea was divided into two states. Although this was not done by Japanese Empire, the cause of it certainly came from her taking away Korean Independence and forming Korea into her colony.

I want to get out of tangent here and say more: Due to this fact, South Korea lacked legitimacy to Koreans in the beginning and it was very unstable state. However, Park Jung Hee, also a Pro-Japanese Collaborator himself, restored legitimacy by having economic recovery as the goal of his government. (In fact, he was a Japanese Imperial Military Officer at Manchuria) He is a person who was responsible of "Miracle of Han River" and current Korean economy; however, due to his controversial past and his dictatorship, he is both loved and hated by many.