Re: What do you think made Sephy the way he is?
Sephiroth thought that he was a Cetra. He was scared when he first found out that he wasnt like Zack, who is human. Then after doing research, he found out that the Cetra had defended the planet from a calamity long ago and most died while the humans had fled and returned after the big fight. Sephiroth figured that it was unfair that humans should only survive, and so he tried to kill everyone and become one with the planet, the way the Cetra was meant to be.
After finding out that Jenova was the Calamity, he realized that she was trying to kill off the foolish people of the universe, and he was against the humans anyway so he continued what his mother had started long ago, for his revenge. At least that's how I see it.