User blog comment:JohnnyN64/Mysteries of Fairy Tail/@comment-117.195.57.105-20140612100501/@comment-7418318-20140612103659

Apart from what FTGM said, also remember that Zancrow learned his magic from Master Hades and Chelia learned hers from a book.

Then there's also the three great fairy magics Fairy Law, Glitter and Sphere. They got their name because of the guild or nickname or something of their creator, so maybe God Slaying magic was created by someone who had a nickname with god in it, or the inventor just thought it'd be a cool and intimidating name.

Personally I'm fine with Dragon Slayer Magic outclassing God Slayer Magic. God Slaying is "just" a regular magic that can be learned by anyone of sufficient talent. It allows you to manipulate your element and stuff, but doesn't change the body.

Dragon Slaying Magic on the other hand is not "just magic". It makes permanent changes to the body, like the increased senses, the elongated canines and motion sickness. It can even lead to the person turning into an actual Dragon under the right circumstance. You can even argue that Dragon Slayers are no longer fully Human.