You and I are walking through a dark alley. You are explaining that it’s wrong to kill an innocent person, but that it’s morally permissible to kill someone who is threatening your life.
At that moment an attacker leaps out of the shadows — and you find that, because of my position, you cannot defend yourself without also killing me.
In that instant, what is your moral obligation? You must not kill an innocent man. But my presence is threatening your life, so in that sense I am not innocent and can be sacrificed.
But if there’s no imperative to spare my life, then I’m no longer a hindrance to your self-defense … in which case I’m innocent again and you’re not justified in killing me.
And so on.