Originally Posted by
Saintheart
This is one of those loaded questions where if one post goes the wrong way, we wind up with a locked thread, so, I will try to proceed with caution.
The only justification for the Cataclysm springs from two things:
(1) The Dragonlance gods are sentient, and imperfect beings but not omnipotent; and
(2) The entire setting is premised on the absolute necessity of good and evil coexisting in the world, which is a pretty weird way to view reality.
The justification for the Cataclysm is given by Paladine: they (or he alone, in some accounts) saw that the Kingpriest of Istar if unchecked would proceed to use the gods' powers to eliminate not just evil, but anything that disagreed with him. After 13 Warnings, and after pulling out all the clerics who still actually believed in their gods (most didn't), the gods hurled the fiery mountain at Istar, destroying it and sending the world into disaster. It also had the effect of removing all current power bases which were essentially dependent on, or followed the views of, Istar. The Solamnic Knights survived but were shamed into exile in pretty well every land except Solamnia itself. It was a brutal way to reset the board, but the gods didn't apparently see any other way to intervene with the Kingpriest and restore the balance. Not to mention that as said, the gods had already pulled out all of their clerics and anyone who still believed and followed the will of the gods (Lord Soth being one who believed in the gods and had the chance to avert the Cataclysm, but purposefully turned away from the opportunity to save the world.)
This justification is pretty contestable from a moral standpoint in our world, but it is internally consistent to the setting - remembering that the setting requires coexistent good and evil and maintains gods who can see the future but are not omnipotent and who are not perfect. So that's the justification: it's required by the rules the setting puts up for itself. This is a separate issue from whether it's icky or genocidal or immoral to throw fiery mountains at people in our own world.