Unable to communicate with their queens, the explorers who originally discovered the rachni deemed them a hostile alien race, sparking a galactic conflict in which the newly discovered insectoid species instinctively spread out to defend their new territory. In order to stop them, the salarians decided to bring up the war-loving krogan to stop their expansion and drive them to extinction, an act that would ironically result in the deployment of the genophage. Now in the year 2183, the fate of the rachni rests in the hands of Commander Shepard. Despite being nearly wiped out in the Rachni Wars, they managed to persist in the form of cryogenically preserved eggs. Finally meeting up with the last queen of their species, Shepard is forced to choose between allowing her to survive and endanger the whole galaxy as a result, or wipe out an entire sentient species. The latter isn't something that should be done lightly, but if they aren't driven extinct here and now, what's to stop them from wiping out the other races of the Milky Way? It's a tough choice, one that shouldn't be taken lightly. Or at least it would, if it mattered in the slightest. Other choices can affect characters and events on a small scale, but even if you decide to exterminate the rachni, they just come back anyway as an enemy type in the third game, undermining a lot of meaning behind this decision. Even though it was shown to be a fruitless decision, it was still one hell of a tough choice, at least for people playing in 2007.