After reading this I came to a conclusion. This guy's an idiot. If he can't understand something then that's bad story-telling? No dude, you're a retard, that's what.
Why Aeries came to save the world at the of FFVII even thought she was dead? Because she became one with the life stream after Sephiroth killed her, everybody that dies becomes one with it, that is their heaven. She was one of those special personsn (an ancient) from an old race that had certain powers that others didn't have and the idea of Sephiroth was to make Meteor do a big enough hole in the planet to become one with the life stream (which is the spirit of the planet Gaia). He thought that he was the most powerful being on the planet and that if he became one with the life stream he could dominate all. You know, like a GOD?
Pretty simple dude but for a retard like you? Well, what can you expect? I mean, Uncharted 2 and MGS4 bad story? You are a J-O-K-E!
Do you even go to school or because you find too challenging you had to drop out?