Jennifer was really in love with Needy.
Ok, maybe this is a leap. Maybe it's not. First off, the best evidence is the BFF necklace and the strange psychic link they seemed to have. It's not a leap to assume the devil or that demon was supposed to give you what you wanted. That was the entire purpose of Jennifer being sacrificed. However, did the ritual fail or mess up, or did the demon get pissed (or maybe the devil got pissed) and instead grant Jennifer with the power to get what she wished. She had everlasting beauty, and the power to get Needy. Perhaps the reason pulling off the BFF necklace did what it did is because the demon in her said "Fuck this, I'm out of here", as what Jennifer wanted was now impossible. Without that, she easily died. On a slightly more insane note, the Evil Dead
shirt. It was clear that Needy was an Evil Dead fan. She had a shirt AND a poster. So, why did Jennifer chose that shirt? Well, she might have chosen it for the awful joke (she's possessed by a demon after a ritual in the woods late at night, and is killing people in gory but funny ways), but there's another possible reason: Needy. Maybe she chose it because she knew Needy would like it.
The demon that possessed Jennifer and gave some of its powers to Needy was the same one used to create The First Slayer
A demon being invoked into an innocent woman, giving her power but making her less than human. Not only is this a good description of Jennifer, but it's a good description of The First Slayer. We didn't exactly see the First Slayer often, but when we did, would it really be a leap to say "Oh, and she eats people." Besides, she's dead. She wouldn't even need to eat. Over time, the powers got further diluted, passing in the same way a bite does it, only with magic. Perhaps a bitten person could pass it along too, but it gets slightly weaker. You do this for millions of years and it's bound to have some kind of effect on the powers, as they would be farther and farther from the source, which has already shown to make them weaker. The reason the First Slayer didn't destroy all of humanity was two-fold. It wouldn't help her, as she'd then starve to death, and she was brainwashed and controlled.
Needy is the second coming of Jesus
Well obviously we have her mother's non non-sequitur dream about Needy being crucified, the physical absence of her father and Needy pointing out Jesus didn't invent the calender (because who better than Him to know, if only subconciously?). The clincher is Chip's line: "I think I already died before you got here but I woke up when I heard your voice".
Following on from this the enigmatic driver at the end is either God or the Devil, depending on how you view Needy's actions after escaping the asylum.
The film does begin with her in an asylum for the criminally insane, after all. Why shouldn't we believe that she's crazy, and that many of the events did not happen as she described them?
For instance, did Jennifer ever even exist? Or is she just Needy's Tyler Durden? Notice, after all, that Needy seems to have some interest in Colin, one of "Jennifer's" victims, while "Jennifer" is obviously interested in Chip, Needy's boyfriend. Maybe Needy simply projected her own libido, her consciousness of her own attractiveness, and her murderous impulses, onto this alter-ego. Also, maybe Jennifer's apparent death actually represented the reintegration of the two personalities. After all, after that event, Needy has "Jennifer's" powers and her murderous impulses.
Or what if Jennifer really did exist, but was innocent of the murders? Needy was in extreme denial about her attraction to Jennifer, which was why she suppressed her sexuality generally. After Jennifer went off with the entire band, Needy started stalking Jennifer and murdering any boy in which Jennifer seemed interested, but projecting blame for the crimes onto Jennifer. Certainly, the authorities attribute the murders to Needy, and she is certainly a murderess at the end.
Alternatively, Jennifer actually did kill people in the story, but Needy is a delusional accomplice.