Just turn it off, there's nothing more.

Yes, humanity was wiped out. The guy in your sig is a ghost, as well as everyone in AC. Boo!

Really though, there are a few theories. Ones I can remember that were not discredited by AC's existence:

Holy was not strong enough to stop Meteor, and Aeris summoned the Lifestream to help it.
Holy destroys Midgar on its way to stop Midgar because, hey, why not.
Holy intended to destroy humanity along with Meteor but the Lifestream stopped them both.