So E.N.D is a fire demon... so Zeref told natsu before that he would have to decide whether or not he wants to destroy it. And even though Igneel failed to destroy it, it's in a dormant state. Also Zeref stated that only Natsu and E.N.D could reach him.
I get a feeling that with all that, E.N.D, being the strongest of Zeref's demons, wants to overthrow Zeref or something. In which case he might even not be all that evil, and even if he is he would have a common enemy with Natsu so Natsu could decide not to destroy E.N.D, just like how Igneel chose not to.
And that finally, it was Zeref himself who one way or the other forced E.N.D into a dormant state.
Just a little theory I thought of xD
Nice chapter. Gray became a ice demon slayer (though I gotta wonder how ice is going to beat fire :P)