I think he knew it was wrong all along, I just don’t think he cared before.
I feel like he had a moment of realisation when he saw the vision of himself saying that he wants to kill everyone. He realised that he wasn't the victim and that he is capable of hurting other people, and that genuinely scared him. That's why he panicked and took to many meds, trying to quickly silence that part of him. Now that he has had time to think he knows that a quick fix isn't possible and that he has actual victims. If he can also become a human being worthy of the person he loves, it's a bonus. That's my reading of it.
I can't tell if Chowon is genuine or not. I am fearing he is forcing himself to appologize because he wants something, and what is hard for him is to have his pride hurt, and not because he really realised what he did was wrong.