User blog comment:Woest/Celestial Spirit King and Zeref/@comment-8153438-20131215082329/@comment-4829046-20131215083000

And like any other "proper" immortal person he had enough of the pathetic state of the world and wanted to die and anticipated Natsu to be the one to do it, same as any shounen protagonist, but as it turned out, he wasn't ready. Then Zeref decided to wait for some more time but finally decided that he will fix the world, no matter how cruel the process is. And perhaps doing that will let him finally die or something, or at least live forever with the satisfaction(and also guilt) of saving the planet.

Which is why Zeref is awesome. XD