Can I sacrifice an opponent's creature to Shadowborn Demon?

Question

I have a Shadowborn Demon and I was a little curious about the description of it. It specifically says "At the beginning of your upkeep, if there are fewer than six creature cards in your graveyard, sacrifice a creature."

So what I was wondering is, do I have to sacrifice my own creature? Or can I try and keep less than six creatures in my graveyard and sacrifice an opponent's creature every upkeep? Since it does specifically says "a creature" — it does not say "your creature" or "a creature you control." Most cards that have a sacrifice involved are extremely specific about that and will say "your creature" or "a creature you control" if it wants you to sacrifice your own creatures.

Answer 1

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.