Koreans used to be an obedient vassal state of China
No they weren't, and the French are the witness to this.
By mid-19th century, France obtained a Qing court guarantee of religious missions across Qing's territory and sent missionaries across Qing. When the French missionaries in Korea were executed by the Korean court because of a ban on Christianity, the French protested to the Qing court why the imperial guarantee was broken. The Qing court replied as follows.
1. Qing's law and edict does not extend to Korea.
2. Korea is a soverign country making its own foreign policy decisions.
3. If France got problems in Korea, then France should take the issue directly to the Korean king.
4. There was nothing that Qing could do to help France on this matter.
So what happened was that the French got duped into the usual exaggeration in China, and that the Chinese word "vassal" does not mean "vassal" in European languages, but "diplomatic recognition".
France then decided to deal with the Korean court directly per the recommendation of the Qing court, leading to the Korea-French War of 1866.
French campaign against Korea (1866) - Wikipedia, the free encyclopedia
The outcome was that the French forces got repelled based on the strength of Korean artillary, which was the best non-European type in region(This is why Korea decided to battle European troops while Qing and Japanese didn't). Korea repeated the same feat with the US troops in 1871, which led the Korean court to incorrectly decide that the Western power could be dealt with and this delayed Korea's modernization by some 30 years.
After SK and NK were formed, SK went ultra nationalism, while still being more or less still a vassal state of USA,
If Chinese call the current Korea-US relationship as "vassal", non-Chinese posters here can understand what the relationship between Korea and Ming/Qing dynasties were like in the old days.
SK hate China's intervention during korean war. Moreover, inferiority complex because of being big countries minion for so many years also generate a lot of hatred.
The fundamental theory of so-called Korean nationalism that is the foundation of North Korea's Juche ideology states that Koreans are superior to all its neighbors.
their feelings of being looked down upon can be too easily provoked.
It is the other way. Koreans look down on Chinese, not the other way around.
Kim Jong Eun can barely stay in power even with China's help right now. So, readers please use your common sense, would he kill north-korean China sympathizers as the article claims at the risk of Chinese back-lash or losing Chinese support entirely?
Yes, because North Koreans see China merely as resource to be utilized, not as equal to them in terms of genetic superiority.
Kim dynasty have been killing all China symphasizers from the 50s, and this will not stop with Kim Jong Eun. Frankly, North Koreans just don't care what Chinese say, and China in turn turnaround and tell Americans that they have no influence over North Korea, so don't tell us to pressure North Koreans, because it doesn't work.
If you need to understand how North Koreans see the world around them, following is an excellent read.
Amazon.com: The Cleanest Race: How North Koreans See Themselves and Why It Matters (9781933633916): B.R. Myers: Books
BR Myers: The Cleanest Race – Re-Examining North Korea | One Utah
Analyzing the propaganda used by North Korea, Myers argued that it is a mistake to consider North Korea a Marxist or Stalinist state. Again and again, he found examples of North Korean propaganda using racial rhetoric not Marxist rhetoric. He argues that North Koreans have spent decades learning that they’re “the cleanest” – i.e. the purist – race on earth and hence superior to everyone else.
So accusing Koreans of "superiority complex" is correct, but the "inferiority complex" is not, especially not against the Chinese.