In Star Trek, the prospect of being assimilated into the Borg Collective—and losing all sense of self—is a fate worse than death. One theory about the evolution of human consciousness could explain why we find the Borg so uniquely unsettling.
"You may encounter Enterprise crew members who've already been assimilated," Captain Jean-Luc Picard says during the film First Contact. "Don't hesitate to fire...believe me, you'll be doing them a favor."
It's doubtful that anyone in the audience disagreed. Writing in Nautilus, Jacob Lopata contemplates whether somewhere, in the recesses of our minds, we sense something unpleasant about our ourselves when we view the Borg. A controversial theory along these lines was proposed by Julian Jaynes, a Princeton University psychologist , in his 1976 book, The Origin of Consciousness in the Breakdown of the Bicameral Mind. He theorized that the ability and tendency to think about ourselves as individuals emerged only around 3,000 years ago. That would mean that anatomically modern humans were alive for hundreds of thousands of years before becoming self-conscious:
Jaynes argues that before this recent emergence of consciousness, humanity experienced the world in a manner similar to the Borg. There was not a holistic self with free will, but rather a two-part psyche, or "bicameral mind," in which one part gave "orders" to a second part that acted on those orders. For bicameral humans, "volition came as a voice that was in the nature of a neurological command, in which the command and the action were not separated, in which to hear was to obey." Jaynes says these commands were often perceived as coming from gods, and that they live on today as the internally hallucinated voices heard by some schizophrenics….As such, the Borg are an excellent example of Jaynes' description of the bicameral mind of early humans. The primary difference is that bicameral humans, unlike the Borg, were not technologically linked together in a single collective mind. Without collective thought, bicameral humans would have had trouble solving and managing complex problems.
So maybe what we really fear is not the behavior of a fictional enemy, but a dark remnant of our historical selves.