Here are my 2 cents on the Sephiroth/ Jenova issue.
Sephiroth's chief issue in his growth and development into a planet-killer has more to do with the duality of his childhood. He was trained to kill to be nothing more then a weapon. He lacked empathy, and so when he found out he was nothing more then a lab rat he freaks. He wants revenge and romaticizes the idea of Jenova as the perfect killer, the mother he never had to bring about his vengeance.