A lot of fans on this site have been saying that the Future Rogue timeline was inverted in the Avatar arc but I'm not completely convinced. These storylines happen all the time. The hero thinks he prevented the future but it ends up finding another way, usually by his actions in another situation. One often meets a certain fate on the road he takes to avoid it. Rogue is very emotionally unstable. What happened to his shadow at the end of Avatar? It couldn't have disappeared. And why would Gray kill Frosch? He didn't kill Natsu, Lucy or Happy, so would he have intentionally? I mean Gray was practically hypnotized by Frosch's cuteness.
Gray could've been trying to kill someone else (hint: Natsu/E.N.D.) and killed Frosch accidentally instead.
Or Gray could've gotten Frosch killed somehow in the Alvarez war and Rogue blamed Gray for it. Like if Frosch was carrying Gray somewhere by air and something happened.
Those are my thoughts. What do you think?