User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-7418318-20150323180532

I just had two new theories on why Gray would kill Frosch.


 * 1) That little guy with a Mr. Cursey got hold of one of Gray's hairs and uses the curse magic to controll Gray's body to do the evil deed. Unlike Kain he also has some way to prevent Gray from talking, so F-Rogue never learned that it was really someone else who "remote-controlled" Gray into killing Frosch.


 * 1) Silver isn't as innocent as he seemed. Either that or during the long time under Keith's control he was corrupted by the Demon's magic.

Keith had control over Silver while he was in his own body. The only way to break Keith's control was to kill Keith, but that had the "disadvantage" of also destroying the bodies reanimated by him. As a back-door into gaining his freedom Silver stored his mind/soul/whatever inside of Gray and made sure that Juvia killed Keith so there'd be noone who could control him.

Once his soul was inside of Gray, he was too weakened to take control of the body immediately, but over the course of the next half-year he slowly subverted Gray's body and mind from within until he was able to make the body his own. To make it harder for anyone to find out the truth and possibly exorcise him from Gray's body, Silver pretends to actually be Gray.