User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-24277053-20150831105531/@comment-26537859-20150831135614

Zeref is basically fighting with his own self. One minute he wants Natsu to kill him, but the next minute he wants to kill Natsu. It's the affect of Ankhersrem Curse. (Correct me if I'm wrong)