The real enemy isn't a physical being, but a software code and the emergent collective behavior that arises from it. That's why the only ethical way -- not to mention the only truly effective way -- to fight the Borg is on a software level: change the program, sever the connections, free the minds.
Something else I'd like to add to this whole debate, if I may be so bold (and it's a very interesting debate, so I hope I'm not striking anyone as rude): While the Borg are much, much more literal than any other case (in that there is
no free will at all and they are truly slaves in every sense), I personally think the
basic concept applies elsewhere too.
If I can mini-rant again (and sorry if it grates, but I'm just trying to describe how I relate to these issues. This is just my
personal take on things):
I remember one of the most important moments of my life- I was 12, and a fight started in the school playground between two children.
As one, the 1,200 other children and adolescents present flocked over to it, leaving me alone. I just watched them all respond in exactly the same way, all moving together and showing the exact same reactions, all eager to shout their support and buy into the collective excitement. It was very sobering. You see, I was being bullied extensively at the time, and all I could think about was "what would be the case if it were widely accepted among adults that I or anyone else
should be bullied- what if such behaviour was actively encouraged at all levels of society?" Well, those 1,200 would presumably move as one just as easily...only against me. That was a very important moment for me, and learning about historical events like lynchings, even the holocaust, didn't shock me
at all after that. But, at the same time, I accepted that the real enemy in such cases was not the individual as such but the social programming that people accept, and a society that
wants its members to buy into ideology rather than embrace individual choice. And so, I also realized that promoting a sense of personal responsibility for your choices was essential- and I mean
real, constant promotion of self-responsibility, not the hypocritical half-assed kind where people pull out the concept for war crimes tribunals and such only to stow it away again afterwards. "Evil" was an action, and the idea and belief that motivated it, not a person (even if some select people became so consumed by the ideas and beliefs they had little else to offer but "evil". In which case, they were to be pitied, and sadly had to be dealt with on a personal level).
Now, let me acknowledge here that of course the people in reality are
not Borg drones- they
don't have the excuse of mindlessness. They
should and
must take full personal responsibility- each and every one, or so I believe. Our individual choices are our own responsibility, and we all have a choice. But at the same time, until human society learns that pressurizing its members into adopting collective ideologies is counterproductive to individual responsibility- well, then these people are hypocrites to then turn around and condemn people for not taking responsibility for their choices! You can't just pull out the "we must all accept personal responsibility" card at your convenience; either you promote full individual responsibility
all the time or not at all (as I hope I've made clear, I'm on the "all the time" side).
Basically, the way I see it, while to encourage individual responsibility is essential, the primary target always has to be the ideological drives motivating everyone's behaviour; disable those, the threat dissipates. I would claim that the only truly effective way to fight
any enemy is to consider ideology and outlook the real foe, rather than the beings themselves (who only embody that foe at present, and move to act on that ideology currently directing them- and who are often in very perilous situations). Viewing the individual beings you oppose as
by default the enemy, inherently evil or inherently the opponent, seems to me counterproductive and dangerous, and only works to fuel spiralling hatred and violence and further acts of evil. It gets you nowhere.
I say, do what it takes to remove the influence of that destructive ideology that threatens your peace; that will always be the most effective way to end a threat. Sadly, in many cases that might well involve killing people, including many who had little realistic choice in the matter (my Grandfather fought in World War II, and of course had to shoot dead many of his German brothers, not because he had any quarrel with them, but because he was responding to the spread of ideologies he felt must be opposed. Each of those deaths was terrible- but the blame lies neither with my Grandfather nor the men he killed, but with the ideologies that had grasped the German nation at the time. In the same way, it was necessary to blow Borg cubes filled with captive drones apart with torpedoes to stop them pounding planetary civilizations into dust). But the primary goal should be to subvert the
idea- aim for the heart, not the hand. Killing drones or enemy soldiers might well be necessary, but that is not the point and I say should never be the point. Try to change the heart, don't just try to lop off as many hands as possible (you might have to do exactly that, certainly, but that shouldn't be the
goal).
In "Destiny", I think Dax and co saw things in a similar way; Picard on the other hand was just obsessed with chopping off as many hands as he could- not a bad idea in theory and
certainly not to be condemned as such (because those hands
were engaged in tearing his civilization to bits, as a lot of people here have stressed! Those hands were engaged in "evil" and had to be stopped). But Dax had a better idea than simply cutting off hands like hydra heads when it was never going to actually work- the only way to win was to tackle the ideological roots driving those hands. Saving the enemy as well as yourself- in my mind, all just war works on the principle "we are fighting to save one another". My Grandfather was posted to Germany in the war's aftermath. He helped rebuild, helped the people there turn away from ideologies that had gripped their nation, and he made good friends there. He was working to save the Germans just as much as the British. And Dax and Hernandez and co saved the Federation by saving the Borg (and the Caeliar). Federation ideals saved them all.
Well...that's how I relate to the ideas we seem to be debating here.
Gods, I can babble on at times can't I?

Again, I hope I haven't come across as rude.