By the end of the Avatar Arc in Fairy Tail’s latest season, many people have complained that it made the Eclipse Portal Arc during the Grand Magic Games seem pointless. The issue is that during the Grand Magic Games, Future Rogue tells Natsu that Gray will kill Frosch. However, in the Avatar Arc, Gray shows no intention of harming Frosch. After some thought, I came up with a theory about what might have happened. Let’s start with the Eclipse Portal Arc.
During the Grand Magic Games, Future Rogue convinces Princess Hisui to open the Eclipse Portal. His plan was to bring dragons to the year X791 to wage war on humans. At the end of the arc, after being defeated by Natsu, Future Rogue reveals his motive. He tells Natsu to protect Frosch one year from now because Gray Fullbuster will kill him. However, Future Rogue’s warning to Natsu wasn’t revealed to viewers until the Avatar Arc.
In the Avatar Arc, Hiro Mashima made it appear that Gray had turned evil and joined ‘Avatar’, a cult that worships Zeref. This led many to believe that Gray would eventually try to kill Frosch. However, it was later revealed that Gray had infiltrated the Avatar cult undercover while working with Erza to thwart their plans. At the end of the arc, Frosch casually approached Natsu and the others, causing Natsu to worry about Gray’s next move. To Natsu’s relief, Gray found Frosch adorable and began petting him. This left many viewers feeling that the events of the Eclipse Portal Arc were pointless, but I think otherwise.
Let’s consider the timeline of Future Rogue and the current Fairy Tail timeline as two separate timelines. When Lucy and Yukino closed the Eclipse Portal and the dragon slayers defeated the dragons and Future Rogue, it altered the future. This means all future events have now changed. Consequently, we don’t know what events occurred in Future Rogue’s timeline.
As for why Gray killed Frosch in Future Rogue’s timeline, the answer might lie in his demon slayer magic. During the fight with Alvarez, Invel, one of the twelve spriggans, tried to bring out the dark side of Gray’s demon slayer magic by killing Juvia. This suggests that his demon slayer powers can take over him.
In the current timeline, since acquiring demon slayer magic, black marks have appeared on Gray’s body. He consulted Porlyusica and learned to control them. Another hint supporting my theory is what Natsu mentioned while searching for Gray. Natsu noted that Gray had those black marks during the fight with Mard Geer of Tartaros and suggested that the demon slayer magic might be taking over him.
Now, imagine that in Future Rogue’s timeline, Gray didn’t consult Porlyusica about the black marks or couldn’t control them. In the current timeline, Gray joined Avatar undercover, but in the other timeline, he might have joined Avatar or another dark guild as an actual member after losing control. This could lead to Gray killing Frosch. Natsu and Lucy would set out to find Gray and face him as an enemy. In the current timeline, Natsu warned Rogue not to leave town, but Rogue, Sting, Frosch, and Lector followed them anyway. Although Avatar was defeated by the time they arrived, in Future Rogue’s timeline, where Gray is an enemy, Sting and Rogue would have to fight him, possibly leading to Gray killing Frosch during the battle. 
There is also another possibility of Rogue engaging in battle with Gray even without Natsu, Lucy and Erza’s interference with Avatar. In the recent timeline, Rogue and Minerva were about to set out for the Avatar related quest before Natsu took it from them. Thus, in Future Rogue’s timeline, even without Natsu and Lucy’s interference, Rogue would have confronted Gray.
What happens next is well known: Rogue is overtaken by his dark powers, kills Sting, acquires his White Dragon powers, and sets his plan in motion to destroy everyone using the Eclipse Portal.
In conclusion, the events were not pointless; they were altered. The future changed when the past changed, and so did the actions of the characters. I believe the story was meant to unfold this way.