Rather than Zeref becoming a villain, he found himself having to play such a role
The strangest things really do occur when you least expect it. Definitely was not expecting an in depth look into Zeref’s past and certainly not any information on Natsu’s origins, but it all makes sense now, how exactly Natsu connects in with Zeref. I now understand why Zeref is fond of Natsu and why Zeref has cast himself into the role of being the annihilator of all humans. Zeref may not desire such an outcome, but the end which can be brought about by such an outcome warrants the need to follow such a path. Zeref wishes to die, but is unable to, the contradictory curse governing him has forced him to walk a path he may not want in order to reach a result he truly desires. The duty Zeref speaks of is of being a villain evil enough so that it inspires a hero strong enough to end him to finally appear – Natsu.
12Dimension 