Yay. Finally. A question I managed to find that falls right into my expertise!

Before, colonization, Korea and Japan were absolutely chummy in every regard. Japanese respected Korean scholars, artists, and skilled laborers as Korea was closer to China during the time and was a great outlet of arts/sciences which were transplanted to Japan. The Wako pirates irritated both countries during early history, but the Wako were not affiliated to either nation, simply nationless bandits that inhabited the rocky islands dotted between Korea and Japan who pirated to survive.

With the Imjin Wars in the late 16th century, this soured the relationships of both countries, with the death of Hideyoshi and usurpation of power by Tokugawa Ieyasu, things rather surprisingly returned to chumminess. Basically, Ieyasu pointed at Hideyoshi's corpse when he took power and said "uh uh, not me, this guy." This is actually the politics that occurred between the two countries during this time in an extreme nutshell, but that is the true meat of the matter.

Koreans also engaged in Sankin Kotai (which was an absolutely ingenious invention), which beneficially worked to deepen trust between both countries, as the premise of Sankin Kotai did not break the rule set in place that armed Japanese were prohibited from entering the Peninsula again.

In this manner, we're going to time skip to the colonization and imperial era.

I'm going to skip right over describing war crimes, because that's a whole different kettle of fish. I believe most educated individuals who shy away from nationalism will understand that any country that conquers another country will have to some degree war crimes.

However, it is in dealing with these crimes that started the bad blood. When the Japanese originally annexed Korea, Korea can be seen as to receive "special" treatment compared to the rest of other colonies. The Japanese Eugenic researchers of the time believed that Koreans and Japanese are closely related (although scientifically speaking, Koreans are closer to Han Chinese than Yamato Japanese). This allowed Koreans to marry Japanese individuals with very little hassle as well as apply for Japanese citizenship. What Japan wanted during the time if things went well was to completely incorporate all of Korea into Japan, removing the different culture, language, and religion.

However, this did not work, and erupted in the Samilsam/Mansei Undong/March First Movement (萬歲運動- 만세운동), where Koreans took to the street to demonstrate for independence. This and after liberation began to sour things immensely for both nations.

When Park Chung Hee instilled his rule as a dictator after the end of the Korean war, he quietly accepted the Treaty on Basic Relations between Korea and Japan in 1965. This included a compensation that Japan entrusted with Park Chung Hee to distribute to his people effected by the war. However, Park Chung Hee used this money to further Korean infrastructure instead of helping victims. The treaty stipulated that any and all actions that Japan grieved upon Korea after Annexation began till the end was herein concluded.

However, this document was kept in absolute top secrecy for 40 years, in which it was uncovered that Japan urged the South Korean dictatorship at the time to assist those effected by the war, but ultimately ignored by the dictatorship.

This and the fact of the Liancourt rocks (Dokdo/Takeshima) also furthers bad blood.

Politically speaking, Koreans tend to be on the extremist side of anti-Japan when needed. Although I cannot pull up any concrete sources, say reputable journal studies or such, it is pretty evident when the Japan hatred starts in Korean news outlets, if you follow Korean news. Usually when something embarrassing or scandalous about the ruling party is leaked, a Dokdo-Takeshima incident almost always conveniently follows, if not, something that isn't really news (like President Park Gun-Hye being a Japan sympathizer for wearing Japanese shoes).

You also have to keep in mind Korea is still trying to keep its national identity. After being cleaved in two after the Korean war and facing change to traditional structures due to Western influences even today, the hatred of the Japanese is the only thing that most Koreans can agree on. This remains particularly true of the older generation that lived through such times.

Japan has taken the opposite approach, mostly ignoring Korean arguments and activities in their mainstream news. Most liberal or state run news also has taken up the practice of hiding the ethnic identity of criminals who turn out to be Korean, unless it is directly relevant to the crime.

Does genuinely bad blood exist? I would tentatively like to say no. I was fortunate enough to go to an Anti-Korean rally in Osaka last year (I am an ethnic Korean but American at heart) and managed to talk to some people who participated in it. I was pretty surprised to find out quite a few people went for the novelty factor of the rally, although the majority did want to oust the Korean minority from Japan (who are not a pleasant bunch).

I cannot speak for Japanese in Korea, but I can speak to a degree for Koreans (outside of Zainichi Koreans) in Japan. If anything, there seems to be a general interest in Korea from the Japanese side, and I have met many Japanese people who can speak Korean better than I can (and I'm someone who dabbles in Japanese/Korean-English translations). I understand that this is what you explicitly stated was not part of your question, but remains wholly relevant, as the best way to experience an existing bad blood is if you go to the "opposing" nation.

On the streets of Japan or even from the numerous average Japanese Joes or Janes (Takeshi and Akikos?) I have talked to, their view of Korea runs from extreme infatuation (K-pop, of course) to general dislike of government. Aside from a few good natured kimchi jokes, there doesn't seem to exist the same amount of dislike of Koreans as olden times, I would even be forced to chalk it as ignorance of their neighbor.

As your question is very broad, if you want to ask or add or refute anything I have written, I would be more than glad to.

Hope this helped.

In Korea there is both a history of resistance to outside invasions such as the Hideyoshi war and the 1905 annexation, and a set of political and social divisions which perpetuate the historical ones.

The older history isn't hard to understand, but after the end of the Japanese occupation the two political systems in the North and South retained vestiges of pro- and anti-Japanese sentiments. The South maintained economic ties to Japan, while the North tied itself to the USSR and China.

The political divisions in the South were and still are mostly regional and class-based; people who felt disenfranchised or discriminated against in the South aligned themselves with anti-Japanese feelings (and the North), while those involved in business took a more pragmatic approach, as did the ruling party.

These divisions have persisted, even as the Japanese threat has faded, so displays of nationalism are still politically necessary.