User blog comment:FairyRaven/Fairy Tail Chapter 424: Avatar, Review and Commentary/@comment-180.191.68.1-20150321133422

Looks like Gray did end up hearing demons in his head, and darkness fell on him. It is said that dragon slayers eventually become dragons when they kill enough dragons. Perhaps the same rules apply to devil slayers. By my count, Gray has already killed off two demons... I guess those voices in his head were getting stronger as the days flew by... :-)