User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-25560094-20150518222949

The last part of chapter "memoirs" makes a sense of me,this part half answers my guess on why acnologia was always chasing zeref. Some part of this series that future Rogue Cheney told to Natsu that in his timeline acnologia was become a ruler of the world,and he also said that Natsu will certainly killed,so assume that acnologia will fight zeref and somehow zeref will be killed (or may be weakened,(like Ophis case in High School DxD),enough for Natsu to beat him),and zeref's powers will stolen, and acnologia uses it to kill natsu and become a ruler.