There are two central arguments in the novel, and they were controversial when the story was first published. The first is that the traditional practice of spoiling the first born son as the most important was stupid. The second is a picture of class warfare, not dissimilar to Marxism. Basically, the story is a Korean Robin Hood, about a powerful, clever, masterful youth who uses his power to steal from the rich and redistribute that wealth among the poor and dying.
The argument is noblesse oblige, since Hong Gildong is a hero because he recognizes his privilege and uses it to help the less privileged. He feels a responsibility to use the power he has been given to help the powerless, and that is quite heroic. That's why even though he is a theft and a criminal lord, the reader still feels on Gildong's side. When he becomes a just king in Yul, that is the ultimate demonstration that his controversial sense for justice is actually correct; he creates a peaceful nation.
The other aspect of his heroism is that he does not prefer people to one another based on birth order or social status or economic status, like his father did. His father regarded him as less valuable because he was second born and because his mother was only the man's concubine. This defined Gildong's sense of justice, and Gildong's reign in Yul is defined by his rejection of that kind of social judgment. He views all people as equal, so he is a noble man.
