
This ending is so complete yet not filling. It was really bittersweet with everything not really finishing but you know the path it's traveling. I thought this story was complete perfection. I think the author wanted the readers to have these feelings so that it makes us feel these feelings and love this story more. I don't know (craving more?) Having a happy ending makes the story have a little weight that it doesn't need. A bittersweet ending causes the reader to have these complicated emotions that just make them crave and love this story.

Yeah i have similar sentiments to astral. Adding on to your question,
Just expressing my own guess: why dad always killed the same type was because he couldn't (successfully) kill his wife for attempting to kill his son. So all those women who died looked like mom. This part is kinda explained when Jin questioned "why not kill mom?"
Why he killed in the first place though, that's still a mystery. I have one possible hypothesis(there are limitless possibilities but just suggesting one): dad had a murderous parent(not pointing to any gender cuz we never know) too. Just like Jin. He didn't manage to escape the cycle (and possibly forgive his parent and/or himself) and it became a habit. Or he was too afraid of going to prison(he rather died than be captured. A.k.a. Suicide).
And unlike Jin, he didn't have a significant other to fully accept him as he was. (mom freaked out and threw jim over and called dad a demon)
So in a sense, the real MVP of this story, imo, is really Kyun. If not it would never end. Maybe. Haha
I really loved the overall story, but even with the final ending, I still feel like there's more. Maybe that's just how I am with all fantastic stories because I don't want them to end, but I really wanted more to be fulfilled. I want to see more into Kyun and Jin's life together, and more with Manny's sister forgiving Jin, and more into a conversation between Jin and his mother, and a reason as to why Jin's father got into killing (and why he kills a specific type), and I just want to read the story of their lives until the very end. This may be selfish since the author already wrote and drew more than she/he had to, but I would really love more. That said, I would probably still be dissatisfied if the "more" story ended, but only because I love this manga so much. Anyways, thank you for your hard work and dedication.