Sephiroth is the result of the foolishness of Shinra in thinking they are above nature. They created Sephiroth using the cells of a creature they couldn't possibly control, thinking they could use it like a tool. But of course the moment everything goes off balance they lose control and their ineptitude shows, and suddenly these powerful executives can do nothing while the ones to finally bring balance back end up being ecoterrorists from the slums.

Sephiroth and Cloud are also mirrors of each other. Both were duped into being tools for capitalist imperialism, when they find out one of them says "I am going to carry on with my own brand of imperialism and conquering" and the other goes (after a lot of character development and a mental breakdown) "I am going to break the cycle."