Here is an interesting theory on why South Koreans hate the Japanese so much:
If South Korea can only weakly legitimate itself through democracy, and with race-nationalism so powerful, Seoul must go head-to-head with Pyongyang over who is the best custodian of the minjok and its glorious 5000 year history. This is a tussle South Korea cannot win, not only because of the North’s mendacious willingness to falsify history, but South Korea’s Westernized culture, massive U.S. presence, rising multiculturalism leading to mixed race citizens, and so on.
The North’s purer minjok nationalism will always have resonance in the South, where for a generation former dictator Park Chung Hee invoked race for legitimacy, 10% of the public voted for an openly pro-North Korean party in the last parliamentary election, and the main left-wing party has consistently equivocated on whether the U.S. represents a greater threat to South Korea than North Korea does.
Enter Japan, then, as a useful ‘other’ to South Korea, in the place that really should be held by North Korea. All Koreans, north and south, right and left, agree that the colonial take-over was bad. The morality of criticizing Japan is undisputed, whereas criticizing North Korea quickly gets tangled up in the ‘who-can-out-minjok-who’ issues raised above. [The National Interest]
I recommend reading the whole article at the link, but likewise the anti-Japanese hatred is irrational when compared to the Chinese as well. The Chinese are actively conducting anti-Korean initiatives because of the THAAD issue, have a territorial dispute with Korea, are the chief benefactor of North Korea, a country committed to the destruction of the ROK, and China was the last country to invade the ROK and nearly destroyed it during the Korean War. Heck the Chinese embassy even sent protestors into the streets of Seoul to beatdown Koreans during the Olympic torch protest.
Despite all of this, hatred is directed towards the Japanese who should be a natural geopolitical ally. I have always believed that the persistent anti-Japanese sentiment and rotating bouts of anti-US sentiment is because South Koreans know they can protest both countries without repercussions. As the current THAAD dispute shows the Chinese government does not sit idly by without retaliating against Korea, likewise for North Korea. If South Koreans push North Korea too much a ROK ship may get sunk or artillery rounds may land in the ROK. Protest Japan or the United States and little to nothing happens. That makes both countries easy targets to direct Korean nationalism towards especially for domestic political reasons.
I don’t expect this dynamic to change unless South Koreans are put into a position where they have to forgive and forget with Japan for national security reasons. As long as the US-ROK alliance this is something Koreans do not have to worry about.