Christine is a Transformer.A very depraved Decepticon.
Christine is from the universe of Cars.Somehow she crossed into our universe, only to find that it is a horrific place where her kind are mindless zombies being controlled by parasitic monkeys. She soon Goes Mad From The Revelation, and suffers a psychotic episode. Hell, I'd bet the Cars answer to Stephen King wrote that exact story.
Christine is inherently evil, she's not possessed.Christine was shown to be evil on the construction line. Surrounded by perfectly normal cars nearly identical to her. There was no horrific accident, no vengeful spirit that met its end in her. There is absolutely no reason for any spirit to be attached to that particular Plymouth Fury. Christine is an entity in and of herself, a Yandere automobile that inexplicably manifests.