User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-7418318-20160928235211/@comment-193.190.73.129-20160929084722

Likely Zeref knows the Etherious can't kill him. That's why he didn't seem all impressed or glad with whatever Tartaros Guild was doing. He even made one Etherious into his bodyguard, rather than a tool to kill himself. These Etherious were made so long ago, therefore they would've killed him by now had they been able to.

This may also be the reason he send Natsu with Igneel in the first place, he knew Natsu didn't have what it takes to kill him. He even outright stated it during the Tenroujima Arc that Natsu was too weak. Or he send him along because Dragons can somehow counter Gods, remember how Natsu shattered that summoned God with one punch and how the scarf he got from Igneel blocked/absorbed Ankhseram's Curse.

Zeref realized he couldn't create something capable of killing himself and that's why his focus shifted from Curses to Magic. It was stated before that Magic is fueled by emotions and evolves over time, so Zeref may have thought that one day a Mage woudl hate him enough or evolve enough to actually pull it off, or something like that.

That's probably why E.N.D. is the last person he had some hope for, as he is proficient with both Magic and Curse power. Whereas the other Etherious rely mostly or solely on their Curses and these Curses likely don't evolve or change the way Magic does.

I do like Nick's idea that Devil Slayer Magic may have been developped by Zeref. After all, Hades once stated that Magic originates from Zeref, thus this may hold some truth.

He likely created Devil Slayer Magic to allow people to dispose of the Etherious and other dmeons he deemed failures. He may have also created God Slayer Magic in the hopes of one day using it against Ankhseram or in an attempt to remove the God's Curse.