I stumbled across this manga by complete chance and I'm glad I did because it is one of the most thought provoking series I've ever read. Despite the MC's lack of typical human emotion I feel so many while reading through, it's crazy that a manga with such a dark tone can invoke feelings sure of sadness, melancholy, and depression but also of hope and happiness even some humorous parts here and there.
On an unrelated note this manga reminds me of another I read long ago that I'm trying to find, the only thing I remember is the MC being somewhat similar in that he interacts with people and tries to fit into society but is also broken and unremorseful about killing. I remember a plot point being that he had to paint a wall in his basement with blood from his victims to keep something behind it satiated and dormant. If anyone knows what it is please let me know!