User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-4449950-20160926123003/@comment-7418318-20160927064516

There is one more thing that would (probably will) bug me about Gray trying to kill Natsu.

Gray was sure that Juvia died, but he spared Invel's life "because it won't bring Juvia back". That was his literal reason for sparing an enemy who tried to kill both him, supposedly succeeded in indirectly killing Juvia and who would have killed a lot more FT mages in he hadn't been defeated. And once he recovered Invel will almost assuredly go back to trying to kill FT mages. And Gray spared him.

Natsu on the other hand is a very good friend who may or may not be very indirectly responsible for Silver's fate... and just like with Invel and Juvia, killing Natsu won't bring Silver back.

If Gray really tries to kill Natsu it would turn him not just into a monumental idiot but also a hypocrit who picks his "principals" on a whim.