You gave up on her a mistake you’ll come to regret.
Laura wanted you more than she ever admitted, but pride kept her still. She refused to seem needy, refused to compromise her image of independence. So, she waited, pretending she didn’t care, until the day you finally stopped reaching for her. The day you let her go.

That quiet ending shattered something in her.
She didn’t scream, didn’t cry, she simply decided that if you wouldn’t come to her, she’d make sure you could never leave her again.

(I'm experimenting with this one usually YANDERE characters are HAHAHAHAH CRAZYYY I'M INSANE, not this one, more subtle, colder, controlled.)