Orson Scott Card
In Visitors, the youthful time-shaping heroes all struggle with the moral dilemma: Is it right to save one world by destroying another? What if that other world is Earth itself?
Meanwhile, if the world can't be saved, what is the point of going on with life -- marrying, having children; or on a larger scale, reforming governments or rebelling against tyrants?
Is it somehow less wrong to kill people who are doomed to die soon anyway? Or is it more wrong because you're taking away the little time they have? Why try to save lives that will be cut short by the Visitors in a couple of years?
And what can be done with the treacherous mice?