User blog comment:Miskos3/Fairy Tail 522 & 523: Gray's Trump Card & Is Destiny Burning, Review/@comment-130.105.140.184-20170218075015/@comment-7418318-20170219093833

When was the last time the parents of an adult murderer were put before a court for not properly teaching their child not to kill?

Personally I see it this way: Zeref didn't create the Etherious to kill Humans in general, but only to kill him. That is the one thing that the Demons have no control over, that they have to kill Zeref. Everything else they chose to do was indeed their choice... or Mard's choice anyway since he was calling the shots.

Thus I don't hold Zeref responsible for all their actions, BUT their actions in the Tartaros arc may be an exception. When Zeref contacted Natsu to tell him "Will it be you or E.N.D.?" he made it sound as if he was well aware of what the Demons did. If he knew they were about to commit a massacre beforehand and didn't lift a finger to prevent it, he is surely sharing in the guilt. If he didn't know and only started watching later, then he's not guilty.