Another option is that the Mirror "Borg" seek perfection along a different path. Rather than creating the best of both worlds between machine and man, these mirror Borg seek spiritual perfection and unity, only using technology to link their spiritual energies (or convey themselves across the cosmos), as they believe that only through unity of thought and spirit will they achieve ascension.
Using this pseudo-psionic network of minds/spirits, they "assimilate" the willing, or those too weak to resist their power, and destroy the rest...the "infidels."
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz