Dean_the_Young wrote...
One one hand you have the demonstrated, recorded history of hundreds of years of contact, warfare, and the last few hours of being attacked by Rachni who have continued to wantonly massacre people.
On the other hand, you have the Queen's words that none of that was their fault, that she isn't directing the Rachni in the labs, and that she'll chart a completely different policy if only you let her go free and not kill her.
...and people say persuasion checks are unrealistic.
A war in which we were the invaders into their space and in which communications seemed impossible to both sides and which they lost. There may also have been Reaper involvement, although that is less certain.
Here on Earth, Germany once seemed to be winning for a while too and were involved in genocide (although we didn't actually even know that until late in the war). Even immediately as of the end of the war, they were considered allied again. We had communications, so they were able to surrender. So far there hasn't seemed to be any negative consequences to our accepting their surrender.
In the Rachni war, we were fighting a race that doesn't even use sound to communicate. and the only means the Queen had to communicate on Noveria was by animating a dying Asari, something one wouldn't be given a chance to do on the battlefield.
The situation is very different now. We are able to communicate. The Queen does know this, does know they lost (and were nearly completely eliminated), and speaks rationally rather than as the kill-crazy race they seemed to be in the war. Communications mean diplomacy is possible. That they lost coupled with rationality means they have incentive to take diplomatic options seriously. And the Queen's value system seemed to value artistic concepts over militaristic concepts, which suggests that they do not have the warrior culture of the Krogan.
There may have been an outside influence too, but even if the Queen is honest about the 'sour note from space' that could still just have been a Rachni call to arms in paranoia over intruders in their space. The point is that they tried once, paid heavily, and know better now. They also have perfect memories, so they won't forget the magnitude of their loss or mercy showed them.
All that is evidence. It isn't conclusive, but neither is a past war conclusive evidence that there will be another.
There is also the fact that there is a common threat comming, and one that eclipses the Rachni. Having another race out there knowing of the threat and standing ready to fight it is a benefit. As the pro Cerberus crowd are fond of saying, in the face of that level of threat sometimes you need to take more risks. Strange that saving the Rachni never seems to be on their list of acceptable risks....