User blog comment:Bluefox87/Chapter 416 Natsu is E.N.D...WHAT?!?/@comment-197.224.47.254-20150120163353/@comment-25572451-20150120171522

The way i read it, Zeref seems to treat E.N.D as a seperate entity from Natsu, like how the Kyuubi has a Yin half and a Yang half in Naruto. Maybe E.N.D is simply just Natsu's dark half hence the reason why Zeref said that it's up to Natsu whether he wanted to destroy E.N.D or not.