In fourth-century Babylon, a man came to Rabbi Rava and said: “The governor of my town has ordered me to murder someone [who is innocent], and has warned me that if I do not do so he will have me killed. [Can I murder the man to save my life?]” Rava refused him permission. “Let yourself be killed but do not kill him. Who says your blood is redder? Perhaps the blood of that man is redder.”
This simple and powerful moral dilemma represents an absolute fulcrum for our moral perspective. On any account of morality based upon enlightened self-interest or the power of rationality (e.g. Korsgaard) we will not reach the proper moral perspective represented by Rabbi Rava: self-sacrifice cannot be justified by reference to the self. So, unless we are prepared to accept these lower forms of morality, we must seek a stronger, even if more uncertain, form of justification.
The idea that his blood is redder is simply code for the fact that we are not worthy to judge. Only God can judge. So we must presume our own unworthiness: we must put ourselves last.