User blog comment:IamJakuhoRaikoben/Chapter 464: Natsu vs Zeref, Review./@comment-24546844-20151130220956/@comment-27276334-20151130222829

I think that the reason Zeref was smiling throughout chapter 464 had to do something with Zeref's hopes of achieving his wish, namely, to die. Perhaps Zeref's conclusion that Natsu has the ability to kill an immortal was influenced by his hopes to die.