In comparative perspective Japanese treatment of Korea and governance of Korea was however bad as it was. There were certainly things that seem nasty and unfair to me as a non-Korean and non-Japanese. Things like the combination of forcing cultural assimilation but still treating the Koreans like they are never good enough. The assassination of Queen Min. Military rule, etc.
But good luck convincing the Koreans that in comparative perspective, the Japanese were competent, and left a decent educational, organizational or physical infrastructure, that the Japanese occupation wasn't the worst thing ever done to anyone in the world by the worst country ever. . For a very universally human reason- they didn't like it, and it happened to *them.*
Hard to say they left decent physical infrastructure when most of the infrastructure was destroyed in the Korean War, which occurred due to the Partition of Korea that in turned happened because Korea happened to be part of the Japanese Empire when they lost WWII.
Education-wise, Korea had a 22% literacy rate at independence, after 3 decades of Japanese rule, and rose to 80% by 1970, so clearly the increase in education attainment amongst Koreans was a post-independence phenomenon. In fact, according to Seung Cheol Oh,
In 1942, only 34 out of 10,000 Koreans were allowed to attend middle school but the number of Japanese was almost 520 out of 10,000. In the case of high school, the number of Koreans was only 2 out of 10,000 while the number of Japanese was 46 out of 10,000.
Plus, the SKY universities (Korea's Ivy League) were also established prior to colonization (by royal decree and missionaries) and were only either co-opted by the Japanese or hampered by Japanese rule.
Organizational, well, all the big chaebols were established after independence. On the Japanese zaibatsu model, sure, but 1. that didn't require colonization for emulation and 2. the social, political, and economic price of having such massive monopolies can't be understated.
In terms of general economics, the Japanese didn't cultivate Korea's local economy or education except to help them extract more resources, same as all the other colonizers.
As per Andrea Matles Savada and William Shaw:
Virtually all industries were owned either by Japan-based corporations or by Japanese corporations in Korea. As of 1942, indigenous capital constituted only 1.5 percent of the total capital invested in Korean industries. Korean entrepreneurs were charged interest rates 25 percent higher than their Japanese counterparts, so it was difficult for large Korean enterprises to emerge. More and more farmland was taken over by the Japanese, and an increasing proportion of Korean farmers either became sharecroppers or migrated to Japan or Manchuria as laborers. As greater quantities of Korean rice were exported to Japan, per capita consumption of rice among the Koreans declined; between 1932 and 1936, per capita consumption of rice declined to half the level consumed between 1912 and 1916. Although the government imported coarse grains from Manchuria to augment the Korean food supply, per capita consumption of food grains in 1944 was 35 percent below that of 1912 to 1916.
There's also the issue that North Korea's cult of Kim seems to have borrowed heavily from Imperial Japan's veneration of the Emperor as a divine entity (such worship and deification was absent from Korean history and other communist countries, so it doesn't seem to have come from those influences) and Korean ethno-nationalistic chauvinism has its roots in Japanese assimilation policy (the term 'minjok' was coined over in Meiji Japan as 'minzoku' and Korean nationalists emphasized the differences between Koreans and the Japanese/Chinese to maintain their identity in the face of Japanese assimilation).
Institutionally, philosophically, and economically, Japan damaged Korea with little benefit after the fact, both since Japanese rule was inherently exploitative and built to exploit more than uplift and because the Korean War ravaged and impoverished the entire peninsula. South Korea's success came after Japanese rule and in spite of it, not because of it, while North Korea still languishes due to the geopolitical conditions that Japanese rule and defeat in WWII resulted in.
Independent Korea likely would've stayed neutral during the World Wars, as it had tried in every major conflict in the 1800s, and so would not have been partitioned and undergone a war that split the peninsula and all the families in it indefinitely.
Really, the only major contribution Japan made to modern Korean success was annihilating the old institutions (the last remnants of the caste system, the dominance of the landed gentry (yangban), the monarchy) so that it could start fresh. And, I suppose, the introduction of modern medicine, which helped increase the population growth rate, but did that really require all the other costs of colonial rule?
Were they the worst colonizer? Well, they weren't quite into population-decimating flavoured genocide as some of the European powers were, so perhaps they weren't the WORST. But saying they were competent, left things behind for independent Korea to benefit from, and the extent of their oppression was cultural and political (not even discussing the sex slavery, which is a common grievance lodged against Imperial Japan that you seem to have omitted) and so were good colonizers is far too generous in the face of what they actually did. At best, they were middle of the pack when it came to Korea. Which says more about the other colonial powers than it says about Japan, really.