User blog comment:IamJakuhoRaikoben/Chapter 355: Song of the Fairies, Review./@comment-3424593-20131010165323/@comment-7418318-20131010173533

I agree. There are plenty possible reasons why they wouldn't want to activate it.

For example they might be planing to conquer the world and END would destroy it. It's a bad idea to destroy what you want to control.

But why do you all assume that END is contained within the book of Zeref?

Walrod said that Tartarus might have the book and I didn't notice any mention of END being contained within it.

I wonder if maybe END is the spell that kept Zeref alive all those centuries. If Igneel tried to destroy this spell and failed it might explain why Zeref recognized Natsu and believed that maybe he could finish what his foster father wasn't able to.