??? What you on about? You got it backwards. He's always been obsessed with her. Now that they've had intimacy, his obsession is just getting worse. The only thing holding him back at this point is the idea of Haesoo hating him for his possible actions. He literally has the power to kill ex and clean up the crime but he's not doing it. She's literally his moral compass right now.
I mean it did showed some backstory of her in the past walking in the rain when he was more younger, he was showing interest from that scene so he’d had always been liking her and plotting on her. She started showing interest in him bc I think it was due to the fact she was very lonely.
I really dont understand how their relationship developed this fast ?? Why is he already obsessed with her when not much has happened