User blog comment:FairyRaven/Fairy Tail Chapter 301: The Dragon King, Review and Commentary/@comment-4047931-20120929202932/@comment-4047931-20120930223410

Well we could assume that about Zeref, as Acnologia was also the human king at the time and Zeref could have been his top Mage, creating spells and experimenting with magic for his king and country. I can only guess that Zeref created evil spells which only brought death and changed Acnologia into what he is now under orders and saw the destruction caused by his actions feeling guilt for what happened because of them and then felt the only way to make up for this was by ending his life. I guess that he wasn't able to end himself so he looks for someone else to stop his sad existence.