
There are several moments where Sangwoo is being charming and people were attracted to that. I'm not sure if there's anyone who like Sangwoo because he murders people though.

You'd be surprised how many people like him for being a murderer... ^^ Besides, for a psychopath, he's actually quite nice. But I'm not expecting people to understand Sangwoo, everyone is too busy pitying Yoon Bum. I am more interested in Sangwoo's history to be honest, because I've read enough stories identical to YoonBum's. Because, after all, most psychopaths (if not all, but I'm not sure) are born from abuse.

No, psychopaths are not born from abuse. That would be sociopath, triggered by trauma events during childhood. Psychopaths are born with their hard wiring different than neurotypicals.
Yeah, I also keep reading Killing Stalking because I am also interested with Sangwoo's character. I am curious to know how the author is trying to interpret his characterization. I think Sangwoo being nice is just his way to blend in the society, after all he is a pro-social. Not all psychopaths are a serial killer, as well as not all neurotypicals are a serial killer too.
Though, I believe that Sangwoo is more as a high-functioning sociopath.
I just finished reading chapter 26 and omg, shit is sad af. I actually cried. Sangwoo is such a bitch honestly. I will actually be shocked if ppl still like him after that chapter.