Suppose you are captured by a man who holds a gun to your head. He gives you another gun and tells you to kill a random stranger before he counts to 10 or he will kill you. From his manner you believe him and his gun is right on the back of your head.
So you kill the next guy who comes along, whereupon the gunman says 'thank you' takes his gun back and walks away.
Does a plea of self-defence get you off for murdering an innocent stranger?