Once again, he just learned that he was a Cetra. He has no emotional involvement with them whatsoever, so his instant reaction of "I will kill everyone in the world!" is bogus.

I have Russian in me. I learned this a few years ago. I didn't freak out and start killing people and committing arson because Stalin murdered a couple million of my distant relatives decades ago. The idea of Sephiroth going from "honorable hero" to "murderous supervillain" by himself, with no help from Jenova at all, in the span of 24 hours, for those reasons is retarded.