Look...Barret. I know...or at least I can guess what you're thinking. If you kill him now then you'll be able to save Gaia. But that's not how it works here. If we die we turn to crystal and we're revived.

[Come to think of it, Cloud isn't entirely sure if this is true of the new group of Heroes, but he knows it's what should happen to Sephiroth...at least he hopes that's still the case.]

If you kill him, or any of us really, we'll just come back. But that might also make things worse for this world. And where will you be after that? It's less than revenge. There's no point.