But that's typically how it is. It's SOP to find out a person's history, why he became who he is or what caused anything if there's any. It's also typical for normal people to feel sorry, pity or empathy on someone who didn't have a nice childhood or past. But it's all part of understanding how a mind works.
the backstory can explain someone's current behaviour but it doesn't exonerate them from any shitty decisions they make. i don't mind finding out a character has a complicated or sad past, i'm irked when the narrative pushes really hard to force the readers to "forgive" based solely on the character's past, disregarding that in the present, they're an asshole. it turns an otherwise ambiguous situation black-and-white. one thing i appreciate about the beast must dies is how it showcases kirin's internal conflict over his attraction to kang moo, because he knows and we the readers know that it's pretty iffy sleeping with someone you suspect is a murderer.
at the end of the day it's just a difference in the quality of the storytelling. writing "unlikable" or "villainous" characters as sympathetic protagonists requires a certain degree of nuance to allow readers their suspension of disbelief. not everyone can be zuko with the best redemption arc ever, but it's still appreciated when a writer tries and noticeable when they don't, expecting their readership to collectively amnesia away a character's bad deeds.
i enjoy reading things with "morally ambiguous" or outright "immoral" characters/content all the time, i'm just really turned off when the story tries to trick its audience into thinking the actual psychopath is a poor widdle baby just by giving him a sob story. like bitch we're not blind or dumb
kang moo on the other hand is unapologetically oFF HIS ROCKER and doesn't give a FUCK, i stan