How did Zeref "Black Wizard" get cursed?
Zeref experimented on forbidden magic which gave him the Contradictory Curse. So the more he values life, the more uncontrollable his power becomes. And when he doesn't value life, the less destructive he gets.
The same curse happened with Mavis who also used the forbidden magic. So basically anyone who experiments or conducts black magic is automatically cursed with the Contradictory Curse.