He always was the red flag . It's just Yujin didn't act out and didn't cause any trouble then. And things weren't romantic it was a deal Yujin was acting his part well. Hunter found it amusing, played along but got attached emotionally. Then he manipulated him into getting into his house but Yujin ran away after meeting his father. The confinement would have happened at some point even if Yujin didn't ran away.
Pretty sure we already knew hunter is deadass obsessed with yujin. He’s been listening to yujin songs specifically 14 track since he’s young and has been ongoing finding yujin even then. Finding yujin during that award ceremony is really just pure luck. What’s different is that, hunter started of with a contract, and he did said, he regretted making that contract with yujin. Coz thats why he couldnt locked on yujin earlier on. So when yujin runaway, hunter true self is shown. Hunter was never shown as green flag. Just like his name hunter, he hunts. Also its a referenced to when hunter talks about his father teaching abt hunting too.
the thing that annoys me with the story rn is how author just changed hunter's personality from a lover boy green flag to a r*pist toxic ml.. and it doesn't make sense bc he rlly seemed like a normal guy.. like the change is so uncanny you'd think he was bipolar