the biggest reason why I prefer South Korea over Japan is a historical one.
I am particularly disgusted with Japan’s history of racism and brutality toward other Asians in WW2 (before before you accuse me of being racist, read this book War Without Mercy, there is a chapter that explains the racial contempt the Japanese harbored toward other lesser Asians, especially the Chinese). I abhor the way they treated Koreans during their 35-year rule of Korea. I am appalled by their group/herd mentality that drove them to commit unimaginably barbaric acts against civilians and POWs in the occupied territories in the Pacific War. I am angry at the fact that they have never sincerely recognized their atrocities such as the Rape of Nanking, Comfort Women, Unit 731, cruelty toward POWs. I have read accounts of the atrocities committed by various empires and colonial powers, including the Mongols, the British, the Americans, but none of those atrocities could rival Japanese atrocities with respect to sheer sadism and brutality. Japanese WW2 brutality is the worst of the worst, much much worse than Nazi brutality (convince yourself with that fact by reading my answers here and here). Most importantly, I loathe the way they mistreated and degraded enemy women. Their cruelty toward women and Allied POWs was legendary and could make the Nazi look like amateur when it comes to torture and humiliation
