Sometimes, it's a way to show the harsh reality of the story's setting. If it's a war - themed novel, killing many characters can depict the horror and destruction of war. Like in 'All Quiet on the Western Front', the large number of deaths among soldiers effectively conveyed the brutality of World War I.
On the other hand, it can sometimes make readers more engaged. They might be eager to see how the story will progress after so many deaths, whether there will be revenge or some form of justice. For example, in a revenge - themed story, the killing of many characters at the start can fuel the protagonist's journey for vengeance, and the readers will be hooked to see how it unfolds.
Not having read the 'killing moon novel', I can only speculate. There might be a protagonist who has a special connection to the moon, either through some sort of power or heritage. There could also be antagonists who are either causing the moon to be 'killing' or are taking advantage of the chaos it creates. And perhaps there are supporting characters like friends or mentors who help the main character on their journey to deal with the situation regarding the killing moon.
Since it's a 'killing star' novel, perhaps the main characters are those who first discover the star. They might be a team of interstellar explorers or astrophysicists. There could also be military personnel if the situation with the killing star becomes a matter of security or warfare. Maybe there's a maverick scientist who has a unique view on how to handle the situation, and a military commander who has to balance the safety of the galaxy with following protocol. These could all potentially be main characters in the story.
I don't know for sure as I haven't read this particular novel. But usually in a killing game - themed novel, there could be a strong - willed protagonist who tries to resist the game's cruel rules.
The main characters might include a range of individuals. There could be a young person who was forced into the horror of the Killing Fields at a young age and had to grow up quickly. Maybe there's also an older figure who has seen the before and after of the situation and tries to guide the younger ones. And there could be a resistance fighter who is determined to end the atrocities.
I'm not sure as I haven't read the book, but it could have a strong, brave protagonist who is trying to deal with the situation on the killing floor. Maybe there's also a villain who is responsible for the chaos there.
Yoon Bum and Sangwoo are the main characters. Yoon Bum is a rather timid and obsessive character. Sangwoo, on the other hand, is more dominant and has a much darker side to his personality.