Can I sacrifice an opponent's creature to Shadowborn Demon?
You cannot sacrifice a creature you do not control. The rule on sacrifice, 701.14a says


  To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.


From a common sense point of view, getting rid of something that's not yours isn't really a sacrifice.