I think this is the first time where I liked every character.

Story
It's unpredictable till the end. I even wanted to look up spoilers but I stopped myself and I'm glad I did.
I predicted who sent the letters and who was the killer but those were easy things, what I didn't see coming was the ending.
I don't know what to say about it. I kinda liked it and I kinda didn't. For all that amazing build up it could have been better.
And that one gigantic plot hole at the end... It didn't ruin the story but it didn't make it better.
First seven episodes are a 10 and the last one is an 8 but I still liked the drama.
It's definitely worth watching and even though the ending could be better it's not disappointing. It gets a satisfying conclusion.
The part about everything being an experiment and the question: Are monsters made or born?
The sins of the kids weren't big, you can't even call those things sins.
So I would say the experiment failed and they are not monsters.
Someone might say they are, maybe it depends on how much we like the characters.
I liked the cinematography and the music, it made the story even better.
Every dialogue and scene is important.
The characters are three dimensional with flaws.
The best thing, there is a bunch of plot twists that make sense, they are not there for the sake of surprising the watcher.
Another thing is that we find out everything with the characters, we are never given more information than them. That means we solve the mystery with them.

The characters
- Moo Yul - the leader. At the beginning he is someone perfect, other's rely on him, they think he will solve everything but soon we see he is not perfect. I liked how even though he was desperate and it was hard, he still helped others when he could just choose to save himself. He has a hero syndrome as the annoying psychologist would say.

- Chi Hoon - the smart guy. At the beginning he seems cold and arrogant, it looks like he doesn't care about others but later we see he is a nice guy. He just thinks logically.
If he was the killer, they would have all been dead in one episode.

The whole group has two braincells and it's those two.

- Young Jae, the bully. He annoyed me the most. He jumps to conclusions and doesn't want to listen to others. He would solve every problem by beating people up. He pretends to be strong and scary when he is not. But he broke down and we saw he was weak, he just wanted to protect himself.
Still not an excuse for his shitty behavior.
The line - It's okay when others hate me but it's different when I hate myself. - That's when I started to kinda like him. Just a little bit.

- Yoon Soo, the quiet guy. His past is messy and his memory is messy. He sees a monster in the corner. He carries a lot of trauma that he didn't properly deal with. He follows what others do and just drops random clues about things.

- Eun Sung, the only girl. She is not happy anymore and she doesn't know why. She first wants to die but then does everything to survive. She is broken and scared but she still flights.

- Jae Kyu, the invisible guy. At the beginning he didn't stand out like the others. He just blended with the background but the more we saw him, the more we found out about him. At the end he was very visible and important.
At some point he looked kinda insane (that scene was amazing) but he came to his senses.

- Kang Mo, the weak guy. At the beginning he seems weak, he gets bullied and suspected by others. In reality he is not weak, he is strong and brave. He makes mistakes but never gives up.

- Mi Reu, the scary guy. I see a pattern, they are shown as something in the first episode but we see their real colors later. This guy is not scary. He is smart and funny.

- Psychologist, I could write an essay about him but I don't like him so I won't.

This drama is awesome. I recommend it for anyone who likes character driven stories and psychology. It's dark and maybe disturbing to some but I liked it.
It will probably stay in my mind for a very long time.