Fridge Logic: If the world would end once the last evil is eradicated, then it appears that the last evil could not possibly be eradicated, because the world ending would be the last evil. Of course, In-Universe, nobody may realize that.
The forces of Good might not even believe (if they knew it was going to) that the world ending in this way was evil, given their other views of things. It's discussed in the novel, where Kaylana says that even assuming they knew, they'd be compelled by their Good natures to fight the evil characters, even if they're saving the world.
True Neutral: The druids, who as keepers of balance sided with the forces of Good and then Evil depending on who was winning. Neither side liked this naturally, and both slaughtered the druids.