I realize that, at the time, the Borg were basically just that, but almost all material about the Borg since then has disproven the idea that they're mindless automatons. Like or hate the Borg Queen, the actual drones still retain the ability to be conscious. They are victims.
Yes they are victims...of the Borg, continuous victims who are turned into a mind-controlled zombie army, no longer able to control their own actions, but are forced to perform as clogs in the Borg's machinery. Only the Borg Queen has any true freedom. And sorry I don't trust her when she claims to some children that being assimilated is "fun" (sure, and I bet her windowless van is full of candy)
This makes their existence even worse, since they are conscious while being mutilated and abused.
Soo YES, they are a mind-controlled zombie army...with the conscious minds trapped in those bodies, suffering every minute as they are helpless against the Collective.
You could maybe argue that ending them would be a mercy, but, well, that argument has been used in real life too with eugenics. Mass slaughter and genocide is difficult to justify, and reducing the Borg to a mindless scourge seems disingenuous... although it is rather necessary for squaring the urge to wipe them out and being able to live with yourself after. It's easier to sleep easy at night if the act you committed wasn't against someone or something redeemable or otherwise "human."
Sure sure bring in genocide/eugenics and the other excuses humans used in history for violence against certain groups.
Except, you know, that argument doesn't apply, since the Borg
are objectively attacking and aggressively subjugating others, it's part of their MO.
And sure there is the problem that the drones can, theoretically, be saved. But you saw how complicated, and time consuming, the process was with the few drones that could be liberated under exceptional circumstances. It's not feasible to do that to every drone. So they it's unfortunate, but they have to bite the bullet so they can't victimize more innocent people.
Sure one day, perhaps, maybe there might possibly a way to mass-deassimilate drones...but when? How many billions/trillions that otherwise would be saved should be sacrificed for that vague possibility? Is anybody in the Federations actively looking for a way to do that? And even if so, what does that mean to a family in the Delta Quadrant that gets assimilated today?
To me Federation objecting to destroying the Borg is the same nonsense as Batman not killing the Joker because he has a personal "no killing" code. We know the Joker is a psychopathic killer who will kill again and we know the Borg are a merciless machine who wills wallow up more people. So from where I'm standing the Federation/Batman are responsible for every victim of the Borg/Joker from the very moment they decided to let them live.
(of course in both examples, the out-of-universe answer is the same. The creators want to keep the popular villain around, so we need to find an excuse for their continued existence, against all logic)
But the Borg were never such an existential threat in the shows that genocide was necessary. It would have been convenient, yes, but unnecessary. Rescuing drones seems a more noble and moral pursuit than killing every drone in the galaxy.
Except they ARE. We see in Voyager that they regularly assimilate entire planets and civilizations. So every day they aren't stopped more innocents have to suffer, more people are mind raped, more cultures are extinguished.
I'm usually the first person who looks for a peaceful solution, but with such an incarnation of Mind Rape as the Borg are, there can be no peace.
Because you know I don't exactly see the Borg Queen listening to reason and going "Oh you are right, from now on Assimilation is on a voluntary basis

"
And again rescuing drones via the current process is not feasible for such a large number, especially since it would be a constant uphill battle. For every 5 drones the Federation liberates, the Borg assimilate 5 billion innocents.