Sry... ╥﹏╥ this buttons r really small
But a kid shouldn't have to be teaching another kid about love and life.
Though you're right, no one else had bothered to teach Yoseob love either. Jaeha had a big heart and was truly loving and kind to Yoseob, enough to try and teach him a new concept.
That's why I consider it mostly the adults' fault especially Yoseob's biological parents.
Yoseob didn't have the parents to make him learn life, just the violence. when they died he gained a monster inside him that replaced his parents, telling him what he thinks his parents would say.
Then Jaeha showed him love. that's when Jaeha started replacing his parents, aka his monster. But Jaeha didn't show him that violence was bad. So Yoseob's inner conflict got worse. He could feel both Jaeha's love and murder intents.
When Jaeha died, leaving only murder intents in Yoseob, Yoseob killed. But he didn't feel good, he needed to ask someone what that feeling was. Jaeha not being here, he transformed him like he did his parents, showing that Jaeha's been Yoseob's monster for far longer then just the end of the story.
Yoseob will never get better. He doesn't understand emotions first because of his parents then because of Jaeha. Him killing people showed he'll never trust anyone again.