They're both people that have been deeply harmed by their surroundings. The MC healed, and Siyun relapses consistently. Two broken individuals on different paths of recovery. The MC is not at fault for assuming the boy who smashed her abuser's head in the wall and has harmed others around her murdered her abuser. She is not and never will be responsible for his actions and she has clearly done all she could to keep him stable, in the same way he is doing all he can to ensure her saftey, despite putting her in danger in the process, and in the same way he came to her mental and physical aid when she needed it the most (when she was actively being bullied, and harming her bully in her dream).
Like others have said, he needs help. She is not to be blamed.
Additionally, the ML underwent severe abuse to become who he is today, he is the product of his traumatic childhood. He is trying to become better. His trauma does not justify his actions, it explains it. The ML being hurt psychologically does not justify attempting to murder Jaehyuk or whatever the char who the MC originally liked was named. The MC originally wanted to murder the people who hurt her too. Luckily, she learned that that was not the path to healing.
What are the comments even arguing? Who's wrong? Who's good? Who's at fault? At fault for what? Their mental place? What is the point in trying to blame them? The MC is wrong for not believing in a boy (who didn't believe in himself) when it came to the murder of someone close to her, who he showed signs of aggression towards a day before, with a tendency for violence and manipulation? Are you kidding me? He DREAMT of murdering him (and by extension, himself). Which means, he absolutely could have.
I didn't expect to see this comments, I'm so baffled.