Oh yeah that's true, but look at it from the Cisharni perspective. You are born in the world and your god creates a beautiful island for you. Over time you note that the countries around you are ruled by brutal systems of government and engaged in petty warfare, so you embark on expeditions to liberate the people and unite the cities into one glorious nation. You share your technology and magic with the people, bringing them into an age of peace and prosperity never dreamed of.
For a few hundred years all is well... until a vengeful god appears, threatening to destroy your city with some cataclysmic disaster. This would end not only your lives, of course, but those of hundreds of common people living aside you. He threatens to kill your families and destroy the greatest beacon of civilization and learning in half the world. Your divine patron intervenes but only manages to talk him into a horrific compromise - step down as the rulers of your country and watch as it falls to war and greed and the stupidity of mortal rulers.
Confronted with such a Morton's fork, what should you do? Well, take a third option, of course. Your god cannot defeat the other one, so you take matters into your own hands. Although your entire people dedicate themselves to this project, your success is by no means guaranteed, and failure means the loss of your immortal lives. But you succeed. You succeed in the greatest feat of strength and courage, willpower and ingenuity, that the world has ever known. You summon the vengeful god in a form that you can actually fight, binding him to the world, and when the dust clears the greatest enemy your people have faced is no more.
At least, you think, until you get word that he has transferred his essence into a small cult he lured from your people, brainwashing them into hatred of everything you have made. You might even find that a close friend of yours, investigating this cult, has been possessed by your ancient enemy, his old personality obliterated and his form twisted into a weapon against you. And they go about, using dark magic to incite your people to madness, to force your generals to fight each other, to starve your people in order to slander your government and gain rebel troops which they instantly plan to sacrifice to fuel their own ends.
I'm not saying that the Cisharni are good (specifically, they're self-important mind-raping SOBs) but you need to realize that with a few small shifts in perspective they aren't the bad guys. And that Cish and Sind aren't any different really, and that the only reason we hate Atlantis is that Sind told us to.
Had we found ourselves in the towers of Atlantis with the entire world open before us, would we be any different than them?