I knew it wouldn't be a happy ending. That said, though, I think LUCA put its message forward in that whoever fights monsters should see that they do not become monsters themselves in the process and to never judge others without knowing their story. I appreciated LUCA for what it was; it would, however, have benefited from exploiting some of its characters' stories a bit more. I would've liked to know more about Lee Son and Choi Yoo Na and what drove them other than the little we saw, but that said, I enjoyed Kim Sung Oh and Jung Da Eun each in their role, they did a great job depicting tattered shadows of their former selves chasing and trying to find acceptance in each other.
As for Kim Rae Won, he delivered as he always does. He perfected the role of a man trying to find and accept himself but was never allowed or even permitted to do so. In the process of fighting to keep his humanity turned into what he hated and fought for, protecting what he held most dear, family. He fully depicted the quote; monsters are made, not born.
Lee Da Hee, what a great actress; I've loved her in everything she's done, and the same with this role. She started strong, but I thought she could've been that much more spectacular given a chance. I look forward to seeing her shine even brighter in her next role.
I loved Zi 0's relationship with Gu Reum; that he was sincere in his love was quite evident, as was she, but in the end, she was nothing more than the very humans, she also tried not to be and like they say, sometimes human places create inhuman monsters. Her lack of understanding or maybe lack of perception of Zi 0 as a person and not a species affected him more than the monsters that hunted them. In the end, it wasn't the monsters that chased after her that got to her; it was the ones that lived in your head. Watch Episode 12 here.