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).
Technically it's not genocide as the Borg was not eliminating their culture or it's people entirely, but instead integrating them into their own culture and using their culture, knowledge, abilities, & technology to better themselves as a whole, and in turn, bettering those they assimilate with the technology and knowledge they already have.
I'm not saying it's right, but it's not the same as Genocide.
Genocide would relate to say for example, all the Native Americans being wiped off the planet / exterminated, just to take their lands and "get rid of a problem" (which almost did happen)
Assimilation is when, as an example, the British won in Canada, and France gave up Quebec to the British. Not only did the British attempt to assimilate the Natives into British culture, but they also attempted to Assimilate the French in Quebec and try to make them adopt the British way of life.
^ That is not Genocide and it is because of this, the show/Borg were particular in using the term "Assimilate"
Your view on what the term "Genocide" means is faulty. Both the French and British were at war many many times over the centuries.... countless amounts of people died along the way, many wrongs were done.... but neither country actually set out to wipe out and kill every living person in either culture.... they each wanted to conquer one another..... when you wipe out an entire civilization, there is nobody remaining to conquer, because you just committed genocide.
In this situation, the Borg were not committing genocide, but if Picard did carry out the action of infecting the entire collective, Genocide would have existed and he would have committed it.
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.
The problem I see with that, is that if this virus was indeed the best choice and it would have worked.... how come, through all the multiple opportunities that existed, nobody else bothered to use this virus against the Borg later on?
Did Picard/Crew destroy their work on the Virus so that nobody else could reproduce it? Normally that sort of stuff gets sent to Starfleet for their own files.
Nobody seems to want to speculate into that concept. Janeway and crew spent a few years traveling through Borg space and fighting them on and off.... you'd think of all people, Janeway & crew would have found through their records the Enterprises' idea of this virus and use it the first chance they got..... then it'd be smooth sailing through borg space and they'd be able to easily take Borg technology and head home in a snap.
The real core of the matter is that you are placing 20th century mentality to a 23rd century situation. Humanity at that time was beyond such acts.... Picard was beyond such acts. His emotions lead him to believe killing them all would be the best solution, due to being personally affected by the Borg over what they did to him, but his principles, the Prime Directive, the Borg's individuality and humanity that still existed and everything the Federation is supposed to be about, says committing genocide and wiping out an entire culture, no matter how evil, is simply wrong and allows the wheel to be turned so that it can be done again in the future..... because it sends the message that it's ok to commit Genocide.
Like I said, if you think committing Genocide is ok, that's your prerogative... however, I differ, and that's my prerogative.