User blog comment:IamJakuhoRaikoben/Chapter 414: Drops of Flame, Review./@comment-5303512-20150107151131/@comment-7418318-20150107194250

I interprete Zeref's actions this way (for now):

Unleashing E.N.D. would very likely result in unpleasant things beyond Zeref's own death, so that's probably why he would have prefered for Natsu to be his killer.

But now that he wants to wipe out mankind, that might no longer matter. Though judging from his words at his previous meeting with Natsu "Will it be you or E.N.D.?" Zeref might still be willing to give mankind a chance and allow Natsu to kill him instead of unleashing END, but his patience seems to be running low.