User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-112.198.90.159-20150119172538/@comment-9595671-20150119181753

If you recall that's when Zeref still valued life and therefore couldn't control his magic. By the time Zeref was with Hades he lost that feeling (hence his eyes going red) so he was able to directly aim it at only Hades.