Sephiroth found out that he's adopted (sort of) and says, "Eh, *** it, all humans must die."
Something to consider here is how Cloud reacted whenever he got close to "Sephiroth" (Jenova). It started off with some telepathic type messages in Junon (shortly after which, Jenova appeared on the ship), and eventually led to self-doubt and mind control. Cloud wasn't chasing Sephiroth or Jenova to the Northern Crater, he was being summoned there. Everyone else went along for the ride.
Sephiroth, by all accounts, was a great guy and a hero up until he went nuts in Nibelheim. Then, suddenly, he lost it. Perhaps it's no coincidence that his sudden lunacy manifested itself when he got within 50 feet of Jenova. He saw some creatures in pods in the reactor, and out of nowhere wondered "IS THIS HOW I WAS MADE TOO???" - kind of a random, unprovoked worry.
I submit this idea was placed into his head by Jenova. Sephiroth never knew who his parents were. His past was somewhat of a mystery to him. Jenova's cells could detect his identity insecurities as a weakness to be played on, just as they saw Cloud's entire image was a facade.
So I don't think Sephiroth made a rational decision to end humanity any more than Cloud made a rational decision to start beating the ***out of Aeris, or to hand over the black materia.