Based on what I've read, it seems that the Greater Japanese Empire treated Korea more badly than the other territories they occupied. They even attempted cultural genocide against Korea by trying to make Korean an extinct language. Now, I know there's at least a little chance that this might not be completely true. But if it is, then why? Were there strategic and/or political reasons for Japan to go so far on them or did they just hate the Koreans very much?
This is a paper that compares Japanese rule over Taiwan to that over Korea.
Notwithstanding the pervasive institution of Japanese-style schools, the uniquely Taiwanese history and culture was not completely stifled by reformed Japanese government and education systems. Taiwanese citizens were still allowed to practice many of their indigenous customs, and although the newer schools were taught in a Japanese dialect, the curriculum still included studies about Taiwanese and Chinese geography, history, and culture.
(...)
Most scholars consider the early Japanese policies in Korea as actually beneficial and even as the catalyst to Korean development and modernization. As mentioned earlier, the first changes implemented by the Japanese was the disintegration of the class system. Moreover, the Japanese abolished slavery in Korea and modified the tax structure to relieve the tax burden suffered by the majority of farmers in the rather agrarian colony (L.o.C., 2004).
After Korea had been officially annexed, Japan implemented further “benevolent” changes in order to quiet Korean resistance to Japanese rule. The Japanese allowed labor unions to organize and began improving the infrastructure in Korea (Vedpuriswar, 2003). The colonial government also developed a manufacturing sector in the highly agricultural colony and enhanced farming technology in order to increase rice output from the farmlands (L.o.C., 2004).
(...)
Korean animosity toward the Japanese increased even more as Japan mobilized Korea for the War effort. Many young Koreans were enlisted by the Imperial Army and many women were forced into the role of comfort women—prostitutes to alleviate the tension among Japanese soldiers. In order to maintain maximum output from the colony, the Japanese adopted a policy of completely assimilating the Korean people. Koreans were forced to change their names for Japanese ones. Koreans were also forced to worship at Shinto shrines. The use of the Korean language was also banned, which in effect shut down Korean newspapers as well. Korean history books were even burned (Vedpuriswar, 2003).
(my emphasis, not the original text)
I wouldn't characterise Japanese rule over Korea as the harshest the Japanese had during their imperialist times (if you count Hokkaido and the Ainu as part of Japanese imperialism, for example). But their actions in Taiwan, with Taiwan being their first important colony, was to make Taiwan "an example of what a colony would be", so there were reasons why the rule was seen as more benevolent than in Korea. Taiwan awas acquired after the First Sino-Japanese War and part of its development was funded from the war reparations paid by the Chinese. Korea was officially acquired after the Russo-Japanese War, and the Russians didn't pay their reparations (the Japanese would acquire the railway in Manchuria as payment, but it wasn't the same as the silver paid directly by China).
In Korea there was much more resistance, because the Korean national identity was much more firmly established (for more than 1,000 years), whereas the identity in Taiwan was kind of established due to Japan separating it from the rest of China.
This resistance is crucial in the difference between colonies. Taiwan colonial governors were always civilians. Korea colonial governors were initially civilians, but after the very important politician Ito Hirobumi was killed by a Korean independence activist, all the following governors were military or retired military officers. So there was not only more resistance in Korea, but the Japanese response towards that resistance in Korea was harsher than in Taiwan.
It must be remembered that Japanese intentions regarding Taiwan and Korea was of making both of them integral part of their territory, with representation in the Diet and so on, but never allow them to be free of Japan. This was not to be repeated with China. Japanese strategy in China was very confusing, with lots of groups and secret societies having their own views, but the most imperialistic of them, which kind of got their way, intended to make the relationship between China and Japan be the same as between India and the UK. To do that lots of puppet regimes would be used, as in Manchukuo, Mengchukuo and Wang Jinwei.
Regarding cultural genocide, and the decision by Japan to forcibly assimilate the Koreans (while at the same time discriminating them, it must be remembered, making them second class Japanese), I'll let to others to answer. Both Taiwan and Korea received the order of changing people's surnames, but implementation was different in both countries. The same happened regarding use of native writing. There was compulsory Japanese schooling in Taiwan, but not the same reaction and conflict over the use of Chinese and aboriginal languages. Probably (sorry for speculation, but there's no other way to answer your question) because there was more armed resistance in Korea than in Taiwan. Armed resistance in Taiwan lasted for about one decade (and was brutally crushed, literally thousands of beheadings happened, and there are pictures to prove it), in Korea it never stopped (and also was brutally crushed, even when it consisted only of peaceful demonstrations).
One interesting fact regarding how "benevolent" the Japanese were regarded in Taiwan was the 2-28 incident, in which Taiwanese tried to rebel against the mainlaind Chinese who ruled a few years after WW2 ended. The people behind the 2-28 incident were Taiwanese students and they used Japanese to speak between themselves incognito, to the horror of the mainlainders who later prosecuted them.
Sources
What do you mean when you say "harsher"? Do you define and quantify harsh in the context of dead bodies of soldiers and civilians? I think defining what is harsh is dependent on what atrocities (ethnic cleansing, sexual violence, mass murder, etc.) are you considering. Take for example the rape of Nanjing, which has been widely downplayed, and still continue to be contested by present day Japanese denialists.