User blog comment:IamJakuhoRaikoben/Chapter 373: Life or Death?, Review./@comment-24551285-20140304133820

Just my guess, I think END is igneel's biological son. So this would make natsu his brother or something. Since it is igneel's son, igneel might not bear to kill END himself, thus igneel couldn't kill END. END used to be a dragon, until he is obsessed by power and when down the wrong path by having zeref turn him into a demon. I think this is why zeref ask natsu to let him live or die as they are both igneel's son and they have the potential to get to zeref.