Not to mention that the Borg, in many ways, can be considered less a civilization and more extensions of a single being. You don't 'declare war' on an individual. If a landing party comes under fire on a planet, they can take all reasonable means to protect themselves, including deadly force, if necessary.
It just so happens that the Borg, due to their nature, are both an individual threat and an extremely hostile one.
Honestly, Geordi's virus was likely the most HUMANE solution available at the time of 'I Borg', since not everyone would have been as easily 'recovered' as Picard was by surgical means, and, even if they were, the logistics would have been far too daunting to even attempt to manually liberate one ship, much less the entire race.
And when you consider the possibilities of other races assimilated in the meantime while the question is dithered upon, which is the greater crime against sentience, really?
I hate when the question of genocide comes up in these debates. I hate it when it comes up in Doctor Who vis a vis
the Daleks... Genocide is one thing when we're talking about a society of individuals who pose no harm to you or the slaughtering of the innocent or murdering people for the crime of being different. But if their very nature is to murder, to kill, and there's not way to placate or reason with them beyond catering to that intent, then diplomacy and ethics do not apply here. You're not dealing with a sentient civilization, then, but a force of nature. Are those innocents slaughtered in the meantime somehow lesser than the ravening, rampaging civilization that's destroying them?
Absolutely, take every measure to avoid the eventuality of destroying them all, if some or all can be saved, but there comes a point where you have to decide who has a greater right to live...
But as I said regarding the Daleks, I do find it tiresome that SF tends to almost always have one of these civilizations merely to spark a false debate on the rights to life of things that have no business living. It doesn't even have a parallel on earth. There's never been a modern human civilization so bloodthirsty that they were at war with everyone, all the time, where no negotiation was possible... Even the go-to society for Evil Civs, Nazi Germany, wasn't THAT bad, and had they been, I'm pretty sure the entire German nation would have been so much nuclear slag by the end of the war.
The fact is, REAL civilizations are comprised of several different kinds of people. Those who support action, those who don't, some who want a third way, etc. Cultures and civilizations deserve the greatest respect and consideration by others while the more hostile parts of them should be worn down through diplomacy so that we can all get along.
The Borg are not a civilization. They are an imposed monoculture, with no variation, and our rules of war, negotiation and ethics do not apply to them.
I know that sounds extreme, but I don't see any way around it.