User blog comment:BlackWizardZeref/Theories on "God Slaying" Magic/@comment-24066070-20131101083700

Most likely the author wanted to create a villain who used fire magic, but since Natsu eats all fire this villain would be pointless. So he decided to invent something stronger than Dragon Slayer Magic and thus he thought of God Slaying Magic. This magic was an utter failure because he allowed the Dragon Slayer to beat the God Slayer while at the same time failing to give a reasonable explanation concerning the origins, characteristics, etc of God Slaying magic.

As if that wasn't enough he even decided to create 2 more God Slayers (one of them being a kindergarten girl) who couldn't beat their Dragon Slayer counterpart. Once again no explanation regarding the origin or purpose of this magic. Apparently the kindergarten girl has mastered this magic to a fairly high degree, while her older cousin doesn't have the magic, so this likely eliminates the possibility of it being inherited from generation to generation.

Bottom line of it all is: the author invented this useless magic due to lack of creativity and all characters who posses this magic failed big time.