The leprechaun King is without a doubt the most vicious and evil creature in the world. This monster has nothing but hate towards the human race and he wants to rule Earth at any cost. He has no problem with killing any human being and he likes to torture them slowly. He lacks any form of empathy towards humans too. This forced the order to hide him because there is a need for this creature alive.
The order has always kept the enemies of humans alive for a very particular reason. If humanity becomes a lost cause, the order will release all of the enemies of humanity and they will no longer defend the species. This would mean that hunting season would be permanent for humans.