What are some terrible facts about South Korea and Japan?
As far as Japan goes, take a look at the torture and human experiments they performed in China during WWI. China actually has museums with statues of Japanese doctors cutting Chinese people open while they are still alive, with no anesthetic, and experimenting on them. There are also sealed glass cages where they put Chinese victims inside to gas them with different agents and see what kind of outcomes they got from different chemical weapons. The Chinese keep these memories alive and fan the flames of hate to this day.
In the 1980s, Korea was getting ready for their first Olympic games, and they wanted to "clean up" the streets, so anyone who looked a bit unkempt was rounded up and sent to labor camps. Many children were taken, and they worked as slaves making consumer products, while the camp owners (many sponsored by protestant churches) made millions of dollars from their labor. The inmates, including the children, were routinely raped and murdered. There were around 30 of these camps in use until a young prosecutor from Busan initiated legal action against "The Brother's Camp", which was one of the most notorious.
Even now, the government has not prosecuted the people responsible for the atrocity. For this reason I view it as still blight on the human rights record of modern Korea.
All nations have skeletons in the closet though, and a few bad actors can tarnish the reputation of a predominantly ok land. People in general are quite humane, but can be led along in mass diversions, like what happened in Germany during the second world war. Emphasizing these kinds of negatives runs the risk of engendering prejudice, and we need to avoid that as we expose and condemn these heinous acts wherever they happen.