Suppose we agree that everyone has a right to life, but that a person forfeits this right when he threatens the life of another–in that case it’s permissible to kill him.
Now consider three people, A, B, and C. A aims a gun at B, B aims a gun at C, and C aims a gun at A. When A takes aim, he’s threatening another person, so he loses his own right to life. Normally in that case C would be justified in killing him, since this defends B. But B is aiming at C, which means he forfeits his own right to life, which means that A can kill him, and that C can’t kill A.

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