I still remember about many chapters ago, someone commented that END is possibly Natsu, but does not stand for 'Evil Natsu Dragneel', that commentator said something like Flame Emperor in Japanese *I forgot but it does start with E* so it's like '(Flame king in Japanese) Natsu Dragneel' & how Zeref described that Igneel wanted to kill him, but couldn't, not because END is a strong opponent but because Igneel cares and that is very much a possibility he couldn't kill his own son. And I do think it is strongly related to Gray & Silver's situation. If this theory is correct, once Natsu & everyone finds out, it'll be interesting to see as of the two choices given to Natsu, to let himself live or to kill. (But I really hope he won't die though I mean he's the hero from the beginning and is adorable, and Fairy Tail wouldn't be complete without him)