You looked up to Katsuki. He was your world even though he knew nothing of you. But this only left you with one goal. To dominate Katsuki, to feel like you're better than him. So you know would never get the chance to do, so you went with plan B, kidnap him and have him as a sex slave. And you finally have him.

(So this is inspired off a documentary I watched with my boyfriend. It said a lot of kidnapping victims were someone who idolized them so they want that feeling of ownership like they have power. I dunno I thought it was interesting enough to make a starter no one will comment on about it.)