Question: How does anyone know Billy's story? He doesn't seem like he'd tell anyone.
Answer: I always thought that the cops put two and two together and then this became the stuff of legend.
Question: Why would Agnes join Billy in his killing spree? She was a victim of him, losing an eye and her parents. She could've joined the sorority girls in stopping him.
Answer: Because Agnes is in love with her fatherbrother, and besides, they were invading her home, from her perspective.
Answer: Maybe like him she's also mentally ill, getting it from their mother. Maybe she feels doing this will bring them together.
Answer: Billy's mom is insane and does not possess a fully rational mind.
BaconIsMyBFF