User blog comment:FairyRaven/Fairy Tail Chapter 427: Pitched Underground Battle, Review and Commentary/@comment-5303512-20150406115839/@comment-7418318-20150406203309

Wait a minute... the covered arm...

Natsu got suspicious of Gray and believed that he was turning Demon the moment Juvia told him about the markings on Gray's arm. And he said that Gray might be corrupted because he got the Demon Slayer Magic too fast.

Could it be that he knows this stuff about Demon Magic because something similar happened to him during the timeskip, only he was barely able to avert negative influence on his mind?