Aggravate, Anger, and Piss off
I'm becoming convinced that E.N.D is Natsu, or at least, a part of Natsu that is not awakened yet, or that has not awakened in a long time. Natsu can choose wether to kill or spare END because it's his own life. If he spares END, then the demon will finally awaken. If he kills END, then he'll be free and just Natsu (a bit like harry being a horcrux). Either he or END will get to Zeref; depends on which one survives.
Also When Zeref first met Natsu he was convinced that Natsu should know him, and more importantly that he should be able to kill him. That would make sense if he were the most powerful demon. Maybe Zeref created END to kill him in the end, after he terminated humanity.
What could E.N.D stand for? Esomething. Natsu . Dragneel.
That would explain why Natsu couldn't get through Freed's age barrier(w.e it was). Not sure about gajeel though.
Hey baby, wanna take a gander at some Adam West penis?
Last week's guess I had. It would be funny if I was correct if Natsu was E.N.D and it stood for Evil Natsu Dragneel. What if Natsu was originally a demon/dragon was transformed into a Human by Igneel, but Igneel couldn't destroy Demon/Dragon Natsu. So instead he stopped it by using some ancient magic that transformed Natsu into a Human Baby making him forget his memories? If not Igneel... Someone had to of stopped it since they didn't succeed.Zeref and Natsu have some type of history... what if they're brothers? Or what if Natsu is actually "E.N.D" "Evil Natsu Dragneel" "Yes that was an ass pull". Natsu and Zeref, have a connection does that mean Igneel has a connection to Natsu parents?