Violently Happy

18 year-old Grace lives alone in her four-story mansion. Her parents died two years ago, leaving her with everything. Even her mothers vibrator. Gross.
She's always secluded herself, never really wanting to interact with people. But, being the rich-kid she was, she was forced to be social very often and was really good at faking emotions. She hated everything about being around people.
That is, until River moved in next door. He was beautiful, dark, charming, mysterious, and secretly psychotic. Lucky for him, Grace was just as secretly psychotic.
She had always fantasized about hurting people, but her fantasies turned more twisted once her parents had died and she spent most of her time alone.