I seem to remember reading an unmade script from Voyager that dealt with the origin of the Borg. They find a space craft and 7 of 9 recognizes the pilot as a member of species 1. He explains the origin of the Borg. His species was developing a planetary defense. He and another man were aboard an orbiting space craft, getting ready to activate the system. When it is activated, the defense system targets its creators and the pilots watch helpless as the Borg assimilate the planet. Trying to find where I read that.