Yeah it's definitely wrong to justify someone's wrong actions using a bad past, but I don't really see that happening here?? The bad past has been given as a REASON for his actions, not a JUSTIFICATION, its so that we can UNDERSTAND him, not FORGIVE him. And as I see it, taehwan is doing everything he can to earn yeonwu's forgiveness, he's giving him space, he's apologising through his actions and he's finally being real with yeonwu. Don't you see?