How many times do I have to tel you guys? SEPHIROTH WAS A PUPPET! He is nothing. Jenova controls his mind. None of his actions were independent. He is just a scared little boy as Sorceress Edea or Ultamecia would say. Sephiroth is a puppet. But this is only fact #1. Onto #2
He wanted to rule the planet. He wanted to be a god. Blowing the planet up was something that Jenova hid from him. She let him think he was going to be a god, but all he would do is kill himself.