In 'I, borg', Picard had a choice:
- either use the virus (and Hugh) and stop the collective (Picard&co were pretty sure the virus will work in the episode - that was the subjective position they were operating on; what was not clearly established was whether the virus would actiually kill the drones or merely dismantle the hive mind)
- or don't use the virus and condemn to death the billions upon billions the borg will kill and assimilate.
In other words, there was no morally 'white' solution.
But Picard&co would be in a clear state of self-defense (or state of necessity, if you consider the borg as not sentient, not endowed with free will, etc) when acting to stop the borg.
BTW, if, hypothetically, a being who could not control his actions attacked you (or another person) and you killed her in defense, you acted in a state of necessity.
Picard choose no to use the virus against the borg, condemning billions - enitre species, whole civilisations - to death.
By acting thus, Picard became responsible, too, for the death of all the billions the borg killed between 'I, borg' and 'destiny',
because he could have stopped all that horror and didn't, knowing what would happen.
But as long as he didn't have to see all the billions dying, as long as he could say that they're not his problem, he could look in the mirror and not see all the blood staining his hands, he could pretend he's the morally white knight in shining armor.
Similarly, in 'Endgame', Janeway had to choose between two options:
- either destroy the transwarp hub, immobilising the borg for, at most, a few years (at most because the borg, with unlimited man-power, could rebuild the transwarp hub; or it could build slip-stream ships; etc), after which time the federation was to be destroyed by the borg (the federation having no chance in hell of stopping even a smallish borg attack - as seen in 'destiny')
- or don't destroy the transwarp hub, keeping the federation a low priority target for the borg, the borg concentrating on other species.
In other words, Janeway had the choice between her peeople and stangers - another situation in which there was no morally 'white' option.
Janeway chose to sacrifice her own people, federation citizens she was sworn to protect - thus betraying her oath as a starfleet officer, to be loyal to the federation.
That's not the kind of stuff I want from watching Star Trek.
Too late for that - Picard&Janeway already did such 'stuff', in the above mentioned episodes (and others).
You just didn't get to see the
billions who did the dying - it happened off-screen (until 'destiny', anyway).