User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-125.212.122.184-20150119222742/@comment-9595671-20150120031043

Natsu did get shocked at the news, but since he didn't seem to have a big mood change I'm gonna say he's not the one who kills him. We can infer someone Natsu knows did it.

Plus here's one detail, F.Rogue didn't mention if it was an accident or on purpose, and when you realize Gray's mission is to kill end... what's to say he may have accidentally hit the wrong target?

Since we don't know the events that went to F.Rogue's time its hard to know what could have happened other then Acnologia's attack.