Guiltythree
My new grand theory: -The world they exist in is built in a grand loop. This is why Sunny's insanity is high (existence of Sin of Solace being stronger) as when the massive ocean loop finishes its cycle, he is doomed to repeat where he started (everything is reset, but Sin of Solace). The truth that drives who know it crazy is that the beginning of creation is also it's end. Thus people who learn that they are living the same life over and over, as they are fated to, wish to break out of it by using the powers of chaos, the corruption. No, I cannot explain the existence of the prior civilizations. But it would perhaps explain the author's desire to emphasize the effects of the whirlpool setting Sunny is in currently, because he's setting it up for the grand reveal at the end of this arc that this is a bigger whirlpool of time.