User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-26287497-20150518155942/@comment-26287497-20150518161448

I may be strecthing it but here goes. Rouge was just jealous that was why he became evil. Plot Twist. Or Someone ambush attack them and instead of Gray dying Frosch got hit and died. Making Rouge go out of control.

Probably? almost same as the ambush attack but this time Natsu is aware of the attck and protects frosch

If all else fails. Either some other method of death Or Frosch wouldn't die.