After binge watching the first part when it got released, I was so excited for the second part like most people!
With a 1month timeskip we saw some character development directly from the beginning when our beloved highschoolers take on missions all by themselves. Without their adults, they quickly adjusted to the dark world and independetly acted in their ways. They showcased many moments of joy and friendship, even though life is hard and got even harder after they lost contact with the military.

So, when everything escalated with a stundets loss and the voting-trust issues, those supressed and pent up emotions were overflowing, since the world is depressing and someday they have to face the hard truth of being left alone. They waited and survived for 1month and after hearing that all their expectations about their CSAT went on vacation, all hope fainted in addition to their trust issues. The cohesion was taken down.

Then when they discovered the corpses of the science students, we all knew that this could particularly happen to our group of students. So their moral or will to fight was weaken even more.

Following the cheerful moments, it already felt like "the calm before the storm" and with the rising tension between yongseo and his inner conflict, the sort of either happy/sad ending was set.

You need to empathize with them! They are Highschoolers, teenagers which were thrown into the cold water so they had to adapt quickly. They have no adults to look up to neither hope insight or a strength group moral. We already saw somebody going mad in part I when another platoon was betrayed by its sergeant and the surviving members being suspicious about the our platoons adults. One student went crazy shooting the vice sergeant... so similar things could have happened with this platoon right?

When Yongseo went on his final rampage, things were pretty realistic (except the ammount of bullets he can shoot with just one mag lmao). A small and closed room with a mass of students and one assault rifle with bascially infinite ammo since it's a fictional series, a massacre was most likely to be expected. That's why I'm not even mad about the author hooping 95% of II parts cast. But, the fact that some survived the first wave of his rampage was even better! They tried to take him down after mag dumping, but also failed in shocked conditions. Unlucky and realistic because if u aren't crazy nor totally emotionaless and shocked u couldn't pull the trigger unless u either done it before or were prepared to do so.

I think many people couldn't handle the end since many ppl wrote "last ep destroyed everything" but let's be honest. What were the options for the stories end unless u read the manwha/webtoon (idk). Happy but boring end, tragic last stand against those alien creatures, in my opinion lame and too cruel. But the final blow throughout the stocked up emotions and students going crazy? In my opinion the best option they had for a dystopia teenage scenario. Teenagers can't handle their emotions :o