In ST:TNG Season 5, episode "I, Borg", the crew comes across a damaged Borg individual that they isolate from the collective, and name him "Hugh".
They develop a plan to show an irrational geometric shape and return Hugh to the collective. Their goal was that the unsolvable shape would, over time, overwhelm their processing power and effectively 'lock up' their systems.
Toward the end of the episode, they decide to give Hugh a choice of asylum or returning to the Collective.
After Hugh chooses to return, Picard hopes that his humanization would spread to the Borg. But he portrayed it as if it was an optimistic hope at best.
Why didn't they go through with their original plan anyway? It wouldn't have changed anything for Hugh since the crew already established that they assumed that if it was discovered by the Borg that Hugh had thoughts or ideas contrary to the best intentions of the Collective, his memory would be erased.
Earlier in the episode, Picard made it clear that since they are not open to peace negotiation and only seek to annihilate the Federation, in Picard's mind any action is justified. (Which of course is somewhat contrary to other Picard lines in different episodes/movies. So this is assuming Picard's current frame of mind.)
I understand he wanted to make an exception for the recently-humanized Hugh, but shutting them all down would still accomplish his goal. He could then find Hugh again, without threat of destruction, and replace his neural processors as they suggested they could earlier in the episode, to undo any overload that the geometric pattern may have caused, and thus easily returning Hugh to a working state again.
Is there some other reason I am missing, or is this a plot hole?