The Borg Part made themselves into cyborgs and starting killing and assimilating everything.
I find it difficult to believe a species would intentionally and willingly make themselves the way the Borg are.
It makes much more sense that they were developing nano-probes and cybernetics as a weapon in a war and that it all backfired on them.
Would you freely have limbs chopped off for implants and give your freedom up to a collective consciousness? I certainly wouldn't, no intelligent species in the Trekverse would neither IMO.