Nowadays, nationalism prevails in Korea and the world. So there are many resulting problems. I think nationalism is a mere obsession, stubbornness in our unconsciousness.
Of course, nationalism advocates say that nationalism can strengthen solidarity. They also say nationalism brings big power and enthusiasm in the World Cup or other big games, which make us unite and feel patriotism. Also, advocates insist that they feel comfortable and glad when meeting the same family (race, tribe) in other country. Eventually, they think nationalism is good for people's emotions and for unification.
In my opinion, however, these are only special cases (World Cup, Olympics, meeting same family...). In everyday life, we still discriminate foreign workers due to our 'single-unity' nationalism. And similarly, people are locked in limited point of view, which can lead to intolerance of others as we can see in Jaebum's case. Also, in an extreme case, Nazis and Hitler killed millions of Jewish people cruelly, only because he thought his race is superior to Jewish people.
For these reasons, I think nationalism is never good for us. We have to break this stereotype. It is better to be a world citizen instead of locked in nationalism.