The Book of Exalted Deeds says that evil means used to prevent greater evil cannot be used by good characters, and says:
Some good characters might view a situation where an evil act is required to avert a catastrophic evil as a form of martyrdom: "I can save a thousand innocent lives by sacrificing my purity." [...] After all, it would simply be selfish to let innocents die so a character can hang on to her exalted feats. Unfortunately, this view is ultimately misguided. [...] What the character sees as a personal sacrifice is actually a shift in the universal balance of power between good and evil, in evil's favor, thus it is not a personal sacrifice, but a concession to evil, and thus unconscionable.
Good ends might sometimes demand evil means. The means remain evil, however, and so characters who are serious about their good alignment cannot resort to them, no matter how great the need.
However, standing by while a catastrophic evil happens, while doing either nothing or attempting a good scheme with very little chance of success, seems to be evil as well.
There is guidance in the book that exalted campaigns shouldn't force PCs to choose between two evils, but what are the characters supposed to do if it happens anyway? Or, what are non-player good characters supposed to in those situations?
Despite all of that text, it still seems that the least evil thing to do would be to actually perform the evil act. Is that the right interpretation?