This is the best example of how a pretentious writer can ruin his own story, by wanting to do a more 'intellectual' work.

I would summarize the problems of this second part in three causes: wanting to make the story darker, the claims to make it more philosophical and transcendent, and wanting to emphasize social criticism a lot.

Regarding the first, it is obvious that the writer wanted to make the story much darker than in the first part, and for this he wanted to resort to all means, even playing the sexual violence card, as occurred in the plot of the prison and in Young Soo's attempted sexual assault on his female comrade in arms. Making a dark story is something that I usually like, but it is necessary to do it with creativity, good taste, and a sense of timing, so that it does not undermine the fundamental purpose of the plot, and here those conditions were not met.

The second was made clear by emphasizing the message that human beings are worse than any alien monster, that there is no worse predator for human beings than human beings themselves. An otherwise irrelevant message, because all adult and moderately intelligent human beings know that sad reality, and if not, there are all the series and movies about zombies to remember it.

And the third is obvious when highlighting the criticism against the South Korean education system and the excessive social pressure on adolescents to succeed in such a demanding and relentless system, and also the criticism against social inequality that multiplies the pressure on someone who comes from a poor family and seeks an escape from poverty in the educational system.

The problem is that by wanting to make those three objectives the main motivation of the second part, the writer forgot that this was basically a fun and exciting science fiction story about an alien invasion and a group of high school teenagers turned, despite themselves, warriors. Everything exciting and interesting from the first part went to waste in the last two episodes, especially the last one, and we were left with a story with airs of greatness that we didn't ask for and that we didn't like.

It's a pity because I liked the first part a lot, and I would give it (in general) a rating of 8 out of 10. Even to the first episode of the second part I would give a rating of 7, and until the penultimate episode I wanted to be lenient, and even the prison plot didn't seem bad at all, although I didn't like the ending very much.

But the last episode ruined everything, I found it to be the worst final episode out of all the hundreds of Korean dramas I've watched. After most of our teenagers managed to survive hard, seeing almost every character we empathize with getting killed by a fellow idiot who suddenly went berserk is so disappointing. Obviously, I would have preferred most of them to have survived the war, but if they had to die, I would have preferred to see them die in battle against the alien monsters, and not killed by surprise by a deranged comrade, something that was done just to send a pretentious message and supposedly transcendent.

I don't want to value the scene of the massacre too much (because it still annoys me) but just to say that after so many battles as veteran soldiers I am surprised that there were not several of them who had the reflexes to quickly repel the attack with their weapons, it seems hardly credible.

After that disastrous ending, having a pre-graduation scene (flashback or alternate fantasy?) thrown in the last few minutes and then a disconcerting post-credits music scene, makes the final episode even worse.

In conclusion, the final episode has been a fiasco, which only deserves a rating of 1 (and that's because the amusement park scenes gave us a little joy). I agree with all the critics, this second part is an annoying waste that ruined a pretty good story in the first part.