Here is the thing, you have an antagonistic view of Jenova. Jenova isn't inherently evil, it just does what it does, destroying worlds is it's purpose. When they see a predator killing a prey in the animal world, most people don't consider the predator to be evil, they will just say something in the lines of "it's nature". It doesn't really differ here: Jenova is a predator and humans are the prey. It's easy to class Jenova as an evil being from an outsider point of view but imagine if it was your mother (or at least if you believed it was your mother, if you know what I mean).
Now that it's said, don't get me wrong, I'm a huge human lover. And if in the end humans defeated Jenova, it just proves that she wasn't superior to us. I just wanted to make my point that Sephiroth isn't insane, his logic naturally derived from the false informations he was given. If you take in consideration Sephiroth's past and the wrong informations he believed, his actions made sense. He was wrong, but he wasn't insane.