There's a podcast called 'Blowback' that's hosted by 2 investigative journalists who do deep dive histories into US conflicts and how said conflicts turned out so poorly for the USA. Their most recent 3 season was on the Korean war. While it's very history dense and not for everyone, if you listen to just the first episode of season 3 you'll get a great understanding of why Koreans harbour a lot of resentment towards Japan. If you like it and keep listening, you'll learn even more.