User blog comment:GotMilk5101520/How will the Tartarus Arc Part 4 will end?/@comment-5303512-20140709034950/@comment-7418318-20140709081704

Zeref never said that Igneel chose not to kill E.N.D. His words were rather ambiguous and can mean either thing. He could have chosen as you said, he could have lacked the power or he might have been emotionally unable to kill E.N.D. From what Zeref said we really can't tell which one is the truth for the time being.