User blog comment:Miskos3/Fairy Tail Episode 246: Tartaros arc: Underworld King, Review/@comment-24546844-20150815140206/@comment-7418318-20150817055739

I don't know.

Maybe Zeref is only hoping E.N.D. can do it, without sure proof? Maybe loophole in the curse? Maybe because curses can kill him, but magic can't?

Or maybe the curse was made in a way that only people Zeref cared for can physically hurt him. With E.N.D. being both like a brother (due to the body of his dead brother being used) and a son (since he created it), the Demon certainly would qualify for such a condition. Considering how cruel the curse itself is, having such a cruel "way out" seems fitting somehow.