User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-76.121.234.236-20150323112525/@comment-24039234-20150323122448

Raven, I think Natsu's logic goes like this: -he doesn't know where Gray is but he's looking for him -he knows Gray will kill the catfrog right about now -so he's going to see what's Rogue up to -conclusion: whatever Rogue will do, Gray's going to be there

I think it's pretty solid. Even the dark guild thingy makes sense, since why would the old Gray kill Frosch? He does not have to know how and why Gray turned evil, but every information clicks nicely in place.