Responses

I agree with you
Yiseon really need someone that can always reminds him that it's not his fault and he's not alone
Hyeon did that, well he's not that green flag guy but he did everything for yiseon
Without Hyeon, yiseon can never step out of his 'home' and the guilt gonna strangle him until he die cause of that
So yiseon need hyeon to live
His sanity is hanging by a thread.... he's already suffering from trauma, survivor's guilt, repressed grief, severe guilt from what he did, fear of abandonment, social anxiety, acute depression, emotional isolation, low self esteem, and suicidal penchants. A psychiatrist would have a field day with his case.
Hyeon is actually doing okay with him. Never rushing or brutalising him, never blaming him for his mistake, taking his side and going with him to his apartment to release the last "undead friend". For some reason, I always imagine him using a gentle tone of voice with him too.
Don't get me wrong, Hyeon is really morally grey. But I think he's good to Yiseon.