When I lived in China, Korea and Taiwan I often heard how some of the natives had an aversion to Japan. A number of Chinese, Taiwanese and Koreans do not talk highly of Japan. I didn't know much about it other than that it had something to do with WWII and comfort women. I was recently doing some research online when I came upon something about Unit 731.
Unit 731 was basically a death camp for Chinese in Harbin, China. They tested biological warfare on humans. It sounded like a pretty gruesome place to say the least. Unlike Germany, Japan never publicly apologized for it's actions during the war. They also never acknowledged the existence of unit 731. Japan's past in the war has been covered up and school children do not learn about this or Japan's other horrible actions during the war.
Unit 731 was commanded by Shiro Ishii. At the end of the war the USA struck a deal with Shiro saying that he would not be imprisoned for his actions if he gave the USA the information on his experiments with biological warfare on humans. It has been stated that some of the same people who were involved with Unit 731 have ties with the Ministry of Education. The Ministry of Education is also one of the ministries that runs the JET program.
Check this out for more information on Unit 731. You may also be interested in hearing about the Rape of Nanking, although beware as this is rather gruesome.