Those things didn't happen in WW2 but in the preceding period. In 1895 Korea was removed from Qing China's protection, up till which point Korea had been a backwater enslaved to the Manchu, and put under Japanese control. 15 years later (1910) it was made a part of the Japanese Empire proper.
Over the next 35 years the Japanese spent untold amounts of sweat and treasure applying the advances of the Meiji Restoration to Korea. However, they also tried to convince Koreans that they were superior Yamato masterrace like the Japanese, in order to justify why Korea was a part of Japan's empire. However, it backfired because Koreans got the message they were superior beings, but forgot the part about "because you are Japanese like me" after Japan was kicked out in 1945.
Basically the reason Korea is so nationalist today is because they were taught so by Japan, and nobody told Korea to stop after WW2 ended.