User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-172.56.3.122-20141216004353/@comment-7418318-20141218105410

Some people think that an unknown past is predestined for some dark secret and since we don't know anything about Natsu before he was already living with Igneel, that keeps the door for such theories wide open. One version of END that some people believe is "Etherious Natsu Dragneel", which is... less stupid than Evil ND (why would Zeref put Evil into his Demon's name?), but still extremely unlikely in my opinion.

And then there's also the matter of language. While FT does have it's share of english loan-words, it is still a japanese manga, so the abbrevation could be for a japanese name rather than an english one.

Or it could even be that it's no abbrevation at all and Zeref only wrote the name as E.N.D. to make it look more important, since he considered this particular Demon to be his salvation or something like that.

I think Lucy = END is just wild speculation on something that the people think would be interesting rather than them really believing it.