User blog comment:Omega natsu2/Fairy Tail Episode 198: The Golden Grasslands, Review./@comment-24039250-20140906071858/@comment-5268982-20140906111058

I just can't shake the feeling that Sting killed Frosch.

Think about it. If Rogue lost Frosch, you would think that he would try to cherish the friends he has left, not kill them as well. What would justify Rogue killing Sting, other than Sting killing his best friend?

Of course Sting has no reason to kill Frosch, but it doesn't have to be literal. Fro could accidentally get in the way of one of his attacks, or try to protect him from an incoming attack, or maybe Sting didn't even remotely hurt Fro, but failed to protect him = kill him in anime language.