User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-1359322-20151207213203/@comment-1359322-20151207215214

Zeref himself already said he can afford to get hit one more time from Natsu. Plus from his wounds and the fact that he lost one of his arm, i think it's quite obvious that Natsu in fact had the chance to kill Zeref in this very chapter.