Who were an enemy army killing BILLIONS of federation citizens each hour. Who were, practically, impossible to save.
Actually, when they decided against using the thalaran weapon, the Borg werent killing anyone, they were travelling on mass back to the Azure Nebula because they had detected the Omega molecule.
The heroes needed a plan to stop the borg once they arrived back there. The point was made that none of the borg drones were responsbile for their actions, they were basically souls trapped in automated bodies, forced to watch the atrocities, and the heroes decided they would not cross the line and use a weapon against them that would be in effect mass murder of slaves. They wanted a plan that not only stopped the Borg but saved the enslaved Borg drones.
And you know, by choosing not to follow the thalaran weapon plan and going with a plan that had the chance to help the drones, they ended up saving all the drones that you called "practically impossible to save".
Beyond that, you talk about the borg killing billions so no mercy should be applied and using the thalaran weapon would slow them down. Surely that is a strange point of view? the thalaran weapon would not have destroyed the entire borg fleet, there were more than four thousand ships there, no way is the thalaran weapon destroying the entire amount before the adapated. So considering that, you use the weapon, destroy a few hundred borg ships, and yet are still left with thousands, who then destroy you, assimilate the OMG (omega molecule generator) and go back and eradicte the remaining Federation and allied worlds. so not only have you used a horrific weapon, banned by all major powers because of its horrificness, you've crossed the moral line for nothing as you havent saved the federation.
The Jem'hadar were also slaves to their genetic conditioning, correct? By your argument, Starfleet had no right to kill in self defense a single Jem'hadar - after all, they had no choice. The Federation should have surrendered, let Weyoun exterminate Earth (as he was planning) and turn all Alpha/Beta quadrant civilizations into slaves. Yes?
The jem'hadar arent slaves in the way the borg drones are, so your point isnt really valid. the jem'hadar are genetically programmed to want to kill for the Founders. the borg drones are enslaved helplessly in their body and forced to watch what occurs by their actions which they have no control over.
And even more, the Federation wouldnt be happy with a plan that wiped out the Jem'Hadar as a species anyway, regardles sof if it offered a tactical advantage, take a look at the reactions of right thinking Federation officers at the revelation that it was Section 31 who engineered the attempted genocide of the Founders.