Jaeha definitely was not doing the right thing, but he was a kid and, as you said, was troubled himself. Yosoeb is more damaged, however, because he wasn’t able to properly develop in his first six years of life due to his toxic environment. The first six years are crucial for a person’s cognitive and behavioural development and heavily impacts your psyche for the rest of your life. Jaeha at least had a loving mother, a normal home life, education, and friends in his younger years that allowed him to develop properly. Jaeha’s only real issue is his insecurity and being bullied in the past, which compounded his insecurity. Yoseob’s very self/mind is his problem. The kid doesn’t even have enough self awareness to know what emotions are or how he’s feeling. Every cognitive association he’s made he’s had to make himself, which is why his cause/effect and perspective on things is completely different from everyone else’s. So while I feel bad for everyone in this story, I see Yoseob as the biggest victim. Everyone failed him, even Jaeha.
Jaeha thought he was doing the right thing.....we can blame him for making Yoseob so dependent on him but he was just a child, and in some ways he's just as dependent on Yoseob too (enough to tolerate dead fish, being strangled every night and having your crush maimed!!)
Yoseob is damaged but so is Jaeha... and at this moment I just don't know which one of them is going to commit the murders...