Why is destroying Fairy Tail explicitly part of GH's mission again? If they know they can just waltz on in, grab Zeref, and get out with virtually no trouble, then why the extermination order?
1. Hades desperately wants to sever any remaining emotional attachment he has to Fairy Tail and his old pupil in order to fulfill his "magic world" goal more efficiently.
2. Hades and GH fear Fairy Tail may have to potential to actually stop them from achieving their goal.
3. For the evulz
4. A secret test of Fairy Tail's ability/conviction/etc.. If they fail, they're killed and wiped off the map. If they succeed (or survive), they're "worthy" of entering the new world GH is planning to create or this all so that Hades might really be trying to motivate them to grow stronger in order to defeat a world-ending menace that only he knows about.