
There's some part in his brain that is just broken/damaged. That part of brain relates with empathy and fear. And if only that part isn't broken, he might have been able to live like normal people.
If you guys would realize this, you would have felt sorry for them and feel grateful that you didn't.
(BTW, that part of brain is called amyglada.)

What even is the point you're making in regards to mine? I am agreeing with you, Sangwoo is a pitiful person with a tragic past, but now he's past the point of no return now that he has tortured and murdered people and felt nothing about it. If you're saying he needs to stay in a mental asylum and never get out of there, then I agree with you. But in context of the story, Sangwoo is guilty and must pay for his crimes one way or another. He can't simply just walk free without receiving his karma.

So is every former combat veteran in the sense that killing regardless of context numbs some to further killing and the PTSD they experience can result in violent episodes. Violence begets violence it's the cycle of abuse. Not only is Sangwoo a veteran, he was beaten and abused to the brink of insanity since grade school and received zero psychological treatment even when having violent blackouts and hallucinations, it would make less sense if he wasn't fucked up. You lacking sympathy for him is more similar to his lack of sympathy towards his victims than you migĥt think, that's the whole point.

Bro. You serious rn? Having to kill in the context of war is not what I’m talking about. I respect soldiers and the shit past and present ones have had to go through for peace. That, and willy nilly murder are completely different. Yeah, I feel sorry for small child Sangwoo, but the current one, I feel no sympathy for. He didn’t ask to be born to a shitty family, but he people he killed didn’t ask to be killed. I feel nothing for him now, he has evened his own “sympathy” playing ground. You don’t see every war veteran out here killing people for no reason.

http://www.mangago.zone/read-manga/killing_stalking/mf/m_chapter-40/pg-46/.
No, we didn't saw him with bruises.Never.Only the mom.
Why tf else would he be happy when the father comes home or be able to live a normal school life?

Sangwoo is a war veteran who was also beaten from birth and witnessed his own mother commit a murder suicide of his father so lacking any formal psychological aid he coped by fracturing his psyche between the violent tendencies of his father and the protective yet obsessive nature of his mother and expressed his emotions in the only way he was taught to his whole life, violence. If you raise a kid to think that clouds are cotton candy, they will, if you raise them to think the only way to deal with their own emotions is to hurt others, they will as well. Young Sangwoo and current Sangwoo are the same person, they are the end product of shared trauma and terrible experiences.
What a sad chapter, everyone accusing and cursing Sangwoo, who's fault is that imagine to survive alone all these hardships without any help.
This is really unfair, give him a slight chance of happiness he's just sick !