a young girl walking through the woods with her boyfriend until she gets the urge to slaughter him. what happens next?

She always wanted to write letters to strangers. She wanted them to be old fashioned, like those messages in bottles or the ones written with inked quills. The anonymity of it all seemed so delightful, the girl would tell them her stories and they would do the same in return. They'd never meet and hear each other's voices but they'd learn many new things together. It was supposed an innocent idea, leaving a letter on a tree for a stranger to see - so why was he so keen on following her?