King Magnifico

While he presents himself as a benign granter of wishes, he only puts up a front as Hope Bringer and only selects wishes that would benefit him directly. When he starts using dark magic, his mask starts to slip and he actively starts manipulating the wishes to suit his needs, granting Simon's wish — to become a Knight in Shining Armor — with the added caveat of mind control-induced loyalty to him.

Example of:
Fairy Devilmother