I said this in another thread already, but i compare Natsu and E.N.D's situation to the Kyuubi's in Naruto. The Kyuubi's yin and yang half are treated as seperate entities, but both referred to as Kyuubi. I actually have a theory. Maybe Natsu was a dear friend to Zeref 400 years ago,so he wanted Natsu to be the one to kill him someday, but maybe Natsu refused to do so since they were such close friends and Zeref in turn sent him forward 400 years to the present time and created E.N.D to fill that role so Natsu didn't have to dirty his hands. But since E.N.D was sealed, maybe the reason why Zeref was so emotional when he saw Natsu was because he had renewed hope that his old friend can take him down.