User blog comment:Omega natsu2/Fairy Tail Episode 198: The Golden Grasslands, Review./@comment-1359322-20140906113410/@comment-24744901-20140906155610

Sting is the most likely choice being as he is the person Natsu would least expect to do it, this would also give Rogue a reason to give up their bond, and kill him.

Lector, uuhhh.... he kills Frosh, because he wants to try and have both Sting and Rogue, as partner, not sure how it would work, but I would imagine his reason would be comedic.

Rogue, he goes crazy after constantly being followed by the shadows, and finally becomes so insane he kills Frosh believing he was one of the shadows following him

Minerva, she kill Frosh because she is a double agent, and has already met Zeref, and this entire time she has been going by Zeref's plan

Natsu kills Frosh because he thinks if Rogue doesn't see Frosh's death then Rogue won't go crazy.

Happy he does it, because he wants to be the cutest cat around, I couldn't think of anything -_-

Lily, he finds out Gajeel has been cheating on him with another cat, and that cat is Frosh, he gets mad with jealousy and kill Frosh.

Gajeel wants to prove that he is the strongest Slayer, and he wants to cut all of the chains of goodness around him believing that they were holding them back, and beats the crap out of Rogue, and Frosh gets in the way, which makes Gajeel mad, mad enough to kill.

Gray he goes crazy form killing demons believing everything is a demon, even Frosh. he then kills him thinking he was slaying a demon. Its hard to think that would happen.

Maybe its lucy, I mean she caused the gates to close, maybe frosh does something that might endanger her friends, IDK what.

Erza gets mad at Frosh because he points out in a joking way how jerza can never be, which gets him kicked into space killing him.

Mard Geer, comes back just to kill Frosh, in front of Rogue, mocking him, which cause Rogue to kill Mard, and go into darkness, and Sting tries to bring him out, but for not as he dies in the process

END, I don't know ._.

Zeref knows about Rogue, and all his possibilities, and figures out that all he has to do to turn Rogue is to kill a cat, which he does.

Acnologia, being bored knows about Rogue, and decides to Frosh for the sake of Rogue to become stronger, wanting to see what would happen if he and him fought in seven years.

One of the top ten wizard saints turns out to be evil and decides to kill Rogue not believing in the goodness of people, but Frosh steps in and blocks a fatal attack killing him.

Jiemma does it to be a douche and kills Frosh to show that he is still more powerful then any of sabertooth could hope to be.

Zero he is breaks out of Brain permanently and begins killing everything, after he some how survives his fatal blow from cobra, which eventually leads him to Rogue and friends, and him killing Frosh in the process.

Well these were situation I could come up with for these characters to kill frosh. Although I think Sting is the most likely killer