User blog comment:B214/Chapter 504: Rift/@comment-27337151-20161001212600/@comment-1776824-20161001231309

Gray is currently still under the belief that E.N.D. was Tartaros' guild master, and if you'll remember, Tartaros was the guild that desecrated his father's corpse and forced them to fight. Gray held on to some belief that Invel was lying, but when he found out that Natsu was indeed E.N.D., to Gray, it seemed like Natsu had been lying to him his entire life. Considering the fact that he's also still in total rage mode from Juvia's death, which he still doesn't know was avoided, this reaction is totally normal. Gray is trying to fulfill the last wish of his father by killing, to him, the originator and source of all of the activities of Zeref's demons. This is a totally natural transition if you look at it logically. And Gray, in his enraged state, cannot rationally process any sort of information. It has literally been, arguably, the worst day of his life up until this point.