User blog comment:B214/Chapter 504: Rift/@comment-4945830-20161002203645/@comment-4945830-20161002210720

You really underestimate how angry it is possible for someone to be at the thought of betrayal, there have been characters in other series that have acted like idiots, or the absolute opposite of a bad guy, and turn out to be evil bastards. Soul Society trusted Aizen for hundreds of years, he betrays them. Whitebeard trusted Teach as a son for years, and we know how stupid Teach can act, sometimes like Luffy, and he betrays Whitebeard. Enma from Reborn acted like a friend of Tsunas, and they were both clutzs, betrays that trust.

It has happened again and again and again. You can say what you say because you are not in his thought process. He is completely enraged from seeing Juvia die, he is not thinking the best, but what he sees is the truth that his enemy told him, and that is a secret he did not know about concerning his "friend." FT is built on trust and friendship and w/e, and seeing Natsu as the one who nearly destroyed FT before will further enrage him.

And as for Natsu being a child when Gray met him, you do know in myth there are demons that can transform and take the appearance of others right Even children. Gray skipped the whole "confusion phase" that goes hand in hand with betrayal because of rage. When someone is pumped up with rage, they tend to act more on instinct. And here is his thought process.

Invel may be lying0> Lets go see Natsu for myself then---> Sees Demon Natsu-> What the fuck is this?!> Demon slaying magic rage fuels anger towards E.N.D.> Attacks Natsu That is the process broken down.