Far be it from me to say what relations between Japan and Korea should be like, but one of the benefits of the existence of nuclear weapons (and I'm not saying that the positives outweigh the negatives, by the way), is that there will never be another Hitler, Caesar, Napolean, Genghis Khan, or... Hirohito again. We can't just go around conquering other countries and being reckless with war like we could before WWII, because nuclear weapons are keeping everyone in check. Hell, WWII and the Cold War forced Western European countries to shift their focus to self-preservation, leading to the decolonization of India and Africa.
That said, Koreans have nothing to fear from Japan.
I am, a bit, curious as to why the Japanese head of state still holds the title of "Emperor." If the Mexican or Canadian head of state or government adopted such a title, I'd consider that an act of war. Maybe that has something to do with it?