User blog comment:IamJakuhoRaikoben/Chapter 477: Transport, Review./@comment-6401212-20160314174619/@comment-7418318-20160314182406

Brandish healing Natsu can be seen as a kind of appology for trying to kill Lucy (twice) for personal reasons, rather than due to the war.

Or Zeref might have told the shields not to kill Natsu. Even without knowing the reason behind that order, naturally such an order would make Brandish very inclined to save his life, since he's obviously somehow significant to her emperor.