First - the borg DO KILL - BILLIONS (genocide by ANY definition) and they assimilate the rest of the population OF THOUSANDS OF STARFARING SPECIES (which would number in the TRILLIONS).
Second - you repeated twice in your post - 'Hindsight 20/20'.
Unlike the politicians who favorized the appearance of nazi Germany, etc, etc, Picard knew FOR SURE that the borg will continue to kill and assimilate. There was nothing uncertain about this.
And Picard&crew also knew that the paradox was the only realistic option to take down the borg, that the individuality program was a long-shot.
Picard had two choices - either let the borg continue their genocidal ways, or stop them with the paradox. All other outcomes had neglijible chances of happening.
These certainties make Picard responsible for effectively condemning to death and assimilation TRILLIONS, just so that he can delude himself he always takes the perfectly moral choice.
Their death and suffering is partly his responsibility, because he could stop it and didn't. Their blood stains his hands, too.