User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-84.151.133.140-20150329085720/@comment-7418318-20150329091603

I can't imagine Frosch being evil. But I could imagine something being sealed up inside of the Exceed and Gray thinks this evil feeling is comeing from Frosch itself and thus he tries to kill it.

But in the end killing Frosch releases the evil. Maybe the evil is even the Shadow tried to take over Rogue all his life, but didn't succeed because it could only exert a part of it's influence from inside it's seal. But once Frosch is dead, it'll take over Rogue full force.