I watched it too (finished it), and my son came along and wanted to watch it as well, but I was like "no, wait, now look away.... and now... well, it's really better if you don't watch it". I thought it was the most brutal, on a physical but more so on an emotional level, film or series I had watched in a long time. Nonetheless I thought it totally worked, it sure kept me watching. Now I just read an article yesterday saying it's the most successful series on Netflix ever... and my son says his friends watched it and also for instance a sister of one, she's like 12.
Well, better to have this discussion when you've finished it, but yeah, it's a good series, but I wouldn't let watch my kid watch it before they are at least 15.
One more thing though, I watched it in English... so you decided to watch it in Korean? While I like to watch some films in original language I have to say I struggle with the Asian ones, because their tone and melody is so different from what I'm used to, that the emotion I get through the sound of the spoken word is not always the one I should be feeling from the story, and that's very confusing.
Speaking of brutality, I watched a German film, Das Zeugenhaus. It's about witnesses of the first Nürnberger trials, who are living in a house together before and during the trial. It's stuffed with famous German actors, and it's told quite suspenseful because at the beginning you don't know about each person's role during the Nazi time, which side they were really on. That makes it an interesting but also a bit nasty viewing experience, in part the actors are so good, and the film has some power, but in a way it also trivializes things by making it a well-told story you like to follow.