User blog comment:B214/Chapter 503: The Last Sight Seen/@comment-5303512-20160926122648/@comment-1776824-20160928165131

Also, for the 800th friggin' time, it is an Etherious' instinct to go after their creator and try to kill him. That is what Natsu is doing. As far as how Natsu is "acting," I refer you to Mard Geer's words during the Tartaros arc regarding the Etherious. Natsu is acting like an animal, and at this point, the only reason for him and Gray to fight is because Gray is in the way of his path towards Zeref. Not because E.N.D. is "evil" or whatever it is you're trying to spin. Natsu clearly recognizes his friend; however, he also very clearly burned members of the Alvarez army alive, if you'll look at the panels. E.N.D. is following his instinct and stepping on, like how Mard Geer also said, any "insects" that get in his way. Literally, just refer to Mard Geer's description of the Etherious and you have every answer for how and why Natsu is acting the way he is.