19. The Borg, Star Trek: The Next Generation, Star Trek: Voyager
Whether it's zombies in horror films or double agents in spy films, villains created from our heroes turned against us are consistently terrifying. Star Trek's The Borg continue this grand tradition into space. While they exist by taking over the bodies of characters we knew and loved, they also strip their victims of their humanity.
The Borg have no interest in negotiations and treaties—they only wish to continue their assimilation of other species. Their only goal is the perfection that can only be achieved through total assimilation. As they often say, "Resistance if futile."
If you happen upon a Borg ship, your best bet is to go warp-speed in the other direction.