Based on chapter 25 , i think Haewon has some Double Personality and the other personality is the assassin one and that personality misunderstood FL by a lot especially since her father was an ass to that side of personality. He might questioned that FL all kindness and sincerity was just fake and all an act. And i bet even in the past life, when Killian came to his sense he was found crying
Brooo i thought when haewon was acting like "omg that guy should redeem himself" When they're in the movie, and or every time he's acting all cute and petite and innocent he's actually changing for the better and trying to redeem himself turns out he's just acting to make the FL lower her guard on him, haewooonnnnnn
Like he's blushing in front of her just so she could see it and made her guard down this guy is totally insane