You target the creature that will be sacrificed when you cast Mercy Killing, then its controller (who may or may not be you) sacrifices it when Mercy Killing resolves. Because this is a sacrifice, neither regeneration nor indestructible will save the creature.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.