WMG / Christine

Christine is a Transformer.
A very depraved Decepticon.
  • Or possibly another Autobot who went insane as a result of witnessing Cybertron's destruction and becoming a space exile.
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.
  • That, or she was possessed right from the start.
    • Well, in the movie thats how it is, but it's never really addressed in the book. Though it seems more to imply that Roland imprinted his rage and hatred upon the car, eventually corrupting it to the point we see in the story.
      • She's not just possessed its like World of Darkness level of spirit...the car is alive in the Umbra..a love/rage spirit whom took on the form of a car.
    • Christine was probably bad from the start, she just needed Le Bay's rage to get her actively moving. Or in other words, she was a very willing and eager vessel for his endless fury.

Christine had a thing for Dennis as well as Arnie.
In the book Dennis decides to sit inside Christine alone while Arnie was buying the car from Roland. Instead of outright killing or hurting him like she did to most people intruding on her; she gives him a vision of her past. In the movie, when Dennis inspects the car, the song, "You keep a knocking but you Can't Come In." is played, cutting out when the lyrics say "Come back tomorrow and try again." pretty much implying that he's welcome to ride in her. She was still being fixed up at the time so to her, it was like walking in on someone while getting dressed. This is backed up later on when Arnie and Dennis go joyriding on New Years, since unlike with Leigh, she doesn't seem bothered by Dennis' remarks. Not even doing a radio scare like she did last time. Even when Dennis was fighting her with the Bulldozer, Christine seemed more fixated with running Leigh down than trying to knock down the Bulldozer.
  • Not just Dennis, but men in general, a vehicular Femme Fatale.

Christine is somehow related to Herbie.
Two cars that have minds and can drive themselves? They've got to be somehow connected. Made by the same process, perhaps? Only Herbie is kind and protective of his owner, while Christine is insane and possessive.
  • Why not a little Maximum Overdrive to go with it? In the 60's, the aliens tried to automate a machine but only succeeded in a car that was almost human. Okay, time to reformulate the creation. In the 70's, a car that is a Yandere. Close, but not murderous enough. Success is found in the 80's with killer vehicles for the entirety of the human race.

Christine was somehow infused with Catoms - Programmable Matter
She is not so much supernatural and more some part of her—like the raw steel that was used to build her body and chassis—was infused with Catoms. Catoms are, in essence, programmable matter that can change its shape and properties at the atomic level. It would explain how she was able to crush herself in order to get at Moochie and repair herself later on. It also explains how she can retain parts that were added like the garage door opener and license plates; she absorbed them and incorporated them into herself. In essence, she's an artificial being. It wouldn't be a stretch for her to be able to actually talk using a smart phone with a text-to-speech app simply laid on the front seat if such technology was available at the time.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/Christine