User blog comment:FairyRaven/Fairy Tail Chapter 328: Zodiac, Review and Commentary/@comment-5980365-20130419163350/@comment-7418318-20130419163836

Maybe that line was just psychological warfare and he "only" intents to subjugate mankind.

On the other hand if he wants to controll the entire world with just seven Dragons, he'll have to drastically reduce the number of Humans so he can easily controll them.