Disclaimer: this review is biased. I have been reading this webtoon ongoing right after it came out several months after I finished reading Bastard. I don't like horror but these authors are amazing! I'm a huge fan of this webtoon and was waiting for a new chapter each week, and even bought some early access. So I was veeeery disappointed in the drama... but if you are a webtoon reader like me, try to not compare this to it, and just enjoy the drama as a standalone, even though i know that it will be hard.

There are spoilers in the review if you didn't read the webtoon. So better to not read it.

Side Note for those who don't like Horror like me: Even though the drama was creepy and disgusting, you could easily close your eyes at the most disgusting parts like excessive blood flow or killings/eatings and still enjoy the show.

STORY:
the Bad:
They changed everything from the very start.. They removed Maria, they completely changed Eunhyuk's sister, they took away Eunhyuk's nerdy side, they added a completely new character who is a strong woman firefighter, They focused more on the outside world, while the webtoon focused only on the residents of this place and from their perspective (the characters didnd't know what was going on, just like us), they changed the group of evil people that barged into their apartment complex, They changed Wook and his profession, they changed Dusik's ending (his end was soooooooooooooooo dramatic and special in the webtoon, i was crying my eyes out but here it wasn't), They removed the "NEW" people, they changed the old man fighter girl duo, They changed the overall ending leaving it open for the second season, they added romance where it wasn't and removed romance from where it was, They breezed through some of the important side characters. Not only that, they also didn't focus much on people's thought process when becoming a monster. The inner fight was less, the chemistry between characters was just shoved down to 0. Eunhyuk Hyun, Wook, Jisoo friendship was my favorite but they just removed it completely. This webtoon was one of the best webtoon with character growth and this was not shown well too. Anyway it just seemed that they are making money on the webtoons name, taking the core idea and doing it completely differently.
The Good:
Now if you take this as a standalone and forget about the webtoon. The characters were interesting, the new firefighter girl was cool and she did a lot for the story progress, the showed us some of the reasons why this started happening making it a big conspiracy. Including military and showing us what they are doing too. So we get more information and the thing becomes much bigger. Some great characters were so perfect that they became scene stealers.

Acting/Cast:
the bad: the bad goes not into the acting but the cast, some didn't suit the characters, like Wook. and some were just made up out of nowhere.
the good: the good is of course the acting, because everyone did a great job with it! amazing performance by everyone!
SOUNDTRACK: Warriors by Imagine Dragons? XD well i like the song so it was nice and epic but kinda unexpected and not very suited.

I never rewatch dramas. So i put a random number in there.

Overall:
Compared to the webtoon, this show is very bad. If i was a webtoon writer i would have been enraged.
But if you watch it separate from the webtoon it is good and interesting.