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My god. There's being an apologist (which you did first) and then there's just being plain ridiculous.
You just said his theme makes sense. That the Planet chose him to kill the Planet and steal all its energy for himself.
So what, the Planet wants to die? It wants Sephiroth to crash a giant rock into it and monopolize all the souls of the dead so he can become a god?
The planet didn't choose crap. Sephiroth is trying to KILL THE PLANET and replace it with his own existence. He is the Planet's arch nemesis just like Jenova was. he knows it and the Plnet knows it.
The theme doesn't make sense anymore.