Nope. Kefka was insane. His mind was raped and altered, regressing him to a childish sadist.
Sephiroth? He went evil ONLY because of what he THOUGHT Jenova did to the humans and what he THOUGHT he was.
But guess what? That was all wrong. He is not a Cetra and it was Jenova who killed said Cetra.
And Sephiroth learned all this as said by him at the Temple of the Ancients. He gained all their knowledge. Knowledge probably including how most of them died and what Jenova really is.
But guess what? Seph STILl wants to be God for no reason.