User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-25111584-20151207131906/@comment-1776824-20151208061357

Zeref was clearly testing Natsu's resolve. He has been holding all that information in for four hundred years, and I'm sure he also at least wanted to die knowing that his brother knew who he was. If Natsu couldn't kill him despite them being brothers, amidst the entire explanation as to why he is in the future if he is END, then he wasn't worthy. I don't see any fallacies in the fight between the brothers, nor do I see any in Zeref choosing to reveal all that stuff to Natsu. If anything, it made the story infinitely more interesting, even if we now have almost no significant mysteries left.