User blog comment:FairyRaven/Fairy Tail Chapter 429 & 430: Code Blue & Purification Ritual, Review and Commentary/@comment-24800928-20150421011113/@comment-7418318-20150421074710

F-Rogue only said "killed" nor "murdered"... and he wasn't the most sane person to begin with. It was probably an accident, a friendly-fire incident or maybe it was one of the Avatar guys and Gray failed to protect Frosch, which F-Rogue's demented mind misinterpreted as Gray being at fault.

If it was one of those three possibilities, it'd also explain why F-Rogue didn't go and hunt down Gray, since some part of his mind might still recognize that Gray wasn't really at fault.