Factual error: The moving truck that transports the Perron family's belongings didn't exist in 1971. It is a 1973 model or newer.
Factual error: The morning after the exorcism, the headlights on all the cars outside of the house are still on. There is no way car batteries from the 1970s would have lasted all night.
Answer: She was a witch that committed suicide. Before, she cursed everyone who set foot on her property.