They were once a normal, albeit technologically advanced faction. Perhaps they were a race of telepaths who were already familiar with joining minds to solve problems, and designed a neural interlink to make the process easier and to sort out random thought processes, as well as add gigaquads of data storage and improve memory, reflex, and recall.
Other implants naturally added to these advantages. But somewhere, the 'humanity' was purged from the borg's collective consciousness. They ceased to think like individuals and focused solely on pragmatic expansion of their network to include all non-networked minds. Perhaps part of their rehabilitation process for criminals... they were forced to join the hive and have thier criminal impulses erased. Eventually, it was decided that all citizens were required to join, to prevent all crime.
Then, there was a war. A species attacked the Borg, because they needed resources. They were a tough, aggressive, powerful race, but they lacked technology. And the hive reacted swiftly and decisively to neutralize the threat. To end the war they were losing so badly, the other species accepted a cease-fire in exchange for being part of the Borg criminal rehabilitation process. They were also linked to the hive and their individuality was erased. Now they were to serve the collective... as tactical drones. Thus the Borg had a powerful military presence.
But their aggressiveness was added to the Borg's collective consciousness...
And soon, all the cold, calculating traits acquired from all of the minds of all of the individuals began to add up, and the traits of warmth and forgiveness, freedom, and desire for recreation began to disappear.
The collective became a ruthless aggressor, with the singular mission of bringing order to the galaxy.... by any means necessary. Thus the Borg are the ultimate police state, a fascist authoritatian regime, a communist collective bent on galaxy-wide assimilation.
Soon thousands of species across thousands of worlds were added to the collective. Some were deemed unworthy, and remained unassimilated. Some few were resourceful enough to avoid being assimilated... but as time progressed, fewer and fewer could withstand the Borg.
The Borg needed a singular mind to command their network, as the collective had difficulty keeping order without a command structure. Even without individuality, different minds had different strengths, different views, different opinions, different past experiences. Someone was needed to silence the masses and have them think as one, someone was needed to speak for the Borg.
I was chosen to be that voice.
For a time, the thoughts of the collective were mine alone. I chose to send scout teams into the Beta quadrant. The Romulans, as they called themselves, were a species worthy of assimilation. However, they possessed technology unknown to the borg; the power to cloak their ships and pass through our sensors undetected. Multiple attempts to assimilate this technology have failed. But it is of little concern, the Borg were easily able to repel their other primitive weapons, and even with cloaking technology, the Romulans were no threat to the Borg. There were other, far more advanced species within our grasp that were worthy of assimilation. Our wars in the Gamma quadrant with the Dominion were less successful, and we were forced to retreat... but the Borg are relentless. We will come again, in force.
We opened a gateway into fluidic space, and found a species most worthy of assimilation. Their biotechnology was far more advanced than ours... plans to assimilate the Federation, a loose organization comprised of several worthy species and hundreds of worlds, were put on hold after they repelled a single scouting cube. We had to commit all of our resources to the war with species 8472.
We had planned to build a transwarp hub in the middle of a sector of space known to the Federation as the "Briar Patch", which is filled with sensor-jamming nebulae, uninhabitable planets rich in resources, asteroids, and other spatial phenomena which make the area quite defensible and hidden.
However, we were in the midst of our war with species 8472, and we were unable to send more than a few dozen vessels into the hub. The federation found out about our plans, destroyed what few vessels we had in the area, and even mounted an offensive against us. We were damaged, but we utilized a temporary alliance with the Federation to neutralize the threat of species 8472.
However, those events along the timeline were erased when the one known as Janeway changed history by travelling back to the Delta quadrant with knowledge from the future regarding ablative hull armor and transphasic torpedoes, as well as a powerful neuralytic pathogen. She managed to destroy my heir, the Borg Queen, and collapse our transwarp network, which severed our links across the quadrants.
I myself am the sole remaining Locutor of the Borg, trapped between the Beta and Delta quadrants. The Borg remaining in my network were undamaged by the pathogen, as several of my drones were from a species with a natural resistance to it, from the Beta quadrant, which is where the pathogen originated. However, they had not had a chance to have their distinctiveness spread throughout the collective. I attempted to disseminate the information through the network as soon as I sensed what was happening, but by then the Queen, who I had left in command of the transwarp hub, had succumbed to the neuralytic pathogen. She finally recieved my instructions, but only after the network began it's collapse.
Now we are a small collective of rogue Borg, and we must assimilate the Beta quadrant. That Romulan cloaking technology might come in handy after all, because we are too few in number to conduct a direct assault into the federation. I will begin construction on a new transwarp hub as soon as we assimilate Romulan cloaking technology to disguise it. We will also begin construction on a massive supercube, with assimilated Federation ablative hull armor and transphasic weaponry.
This time there will be no stopping the Borg. Resistance is futile.