Using the Cthulu expansion, I had a card called Corruption that reads "Draw a madness card. Destroy a minion."
Does this mean destroy any of your minions, any minion in play, or a minion in your hand?
The phrase "a minion", without any further description, refers to any minion in play, no matter its power, base, faction, owner, etc. It does not refer to any minion in the deck, hand or discard pile.
Also, you cannot destroy a minion in your hand, because the term "destroy" means to remove a minion from in play and place it in the discard pile. It's like trying to burn a book that hasn't been printed, or eat a sandwich that hasn't been made yet.