Mai Chazit - Whose Blood Is Redder
The gemara in Sanhedrin asserts that piku'ach nefesh is 'docheh' (overrides) all the mitzvot in the Torah. Hence, if a gentile were to threaten to kill someone unless he violates an aveira, that person should transgress the aveira (under normal conditions) and not sacrifice his life. There are, of course, three exceptions to this rule, known as the shalosh chamurot – the three cardinal sins of avoda zara (idolatry), giluy arayot (adultery) and shefikhut damim (murder), in which instance piku'ach nefesh does not apply and a person is required to surrender his life and not violate the aveira. The gemara locates a source for this halakha - known as "yeihareig ve-al ya'avor" - regarding retzicha (murder) and then extrapolates from retzicha to giluy arayot. If someone threatens to kill someone unless he murders another, it is understandable that one should not spare his life by taking another. As the gemara asserts, "mai chazit de-dama didach sumak tefei" (who says your blood is redder than his?). Unable to render judgment as to whose blood is "redder," one is forced to die rather than take another life. As the gemara states, no pasuk is necessary to determine yeihareig ve-al ya'avor in the instance of retzicha; the above stated logic is self-sufficient.
Rav Chayim, in his commentary to the Rambam (Hilkhot Yesodei Hatorah), suggests two different ways of understanding the logic of mai chazit. Conceivably, the gemara might be claiming that, given the inability to evaluate and compare different lives, a person must remain inactive and not shed another life – for perhaps that life is more valuable. Theoretically, if there existed a meter to gauge the respective worth of the two lives, and one's life were discovered to be more valuable than anothers, he would then be allowed – or even mandated - to murder so as to preserve the more precious life. Being that one cannot effect this judgment, one cannot kill to save his own life.
An alternative approach would suggest that the mai chazit logic merely claims that piku'ach nefesh does not apply when the aveira of retzicha is being transgressed. Even though piku'ach nefesh warrants the violation of all aveirot, it does not mandate murder. This might be based upon the inherent severity of murder (parallel to the inherent severity of avoda zara). This explanation, though, poses a distinct problem: how does it accommodate the gemara's terminology - "mai chazit..."? If piku'ach nefesh simply does not apply to the sin of murder, why does the gemara provide this verbose logic? It should have asserted a more succinct explanation!!
On the other hand, the inability of piku'ach nefesh to override the sin might be based not upon the severity of the sin of murder, but instead upon the inevitable loss of life in such a situation. Generally, piku'ach nefesh's supremacy stems from our desire to prioritize life (and sustained fulfillment of mitzvot) even at the expense of the transgression of a single aveira. However, when that aveira itself entails loss of life, piku'ach nefesh does not apply (since one life will be lost anyway). See especially Rashi's formulation in his commentary to the gemara in Sanhedrin (74a).
These different approaches in explaining mai chazit yield an interesting nafka mina: Would yeihareig ve-al ya'avor apply to a sin performed passively? One of the Ba'alei Tosafot (Rivam) adopts an opinion (cited in several Tosafot in shas - Ketubot 3b and Sanhedrin 74b) that if a woman remains completely passive, she has no obligation to surrender her life to avoid giluy arayot. As the very source of yeihareig ve-al ya'avor stems from the logic of mai chazit in the case of retzicha, passive involvement is not included in this principle. I cannot actively murder since his blood might be redder. But by similar logic, if I am completely passive (for example, I allow myself to be thrown onto another person whereby he will be crushed), I cannot refuse, since my blood might indeed be redder than his. The paralysis which results from the inability to evaluate forces me to withdraw from any active decision. If the murder will be executed without my active contribution, the logic of mai chazit disappears and yeihareig ve-al ya'avor no longer applies.
Conversely, according to the second reading of mai chazit, piku'ach nefesh simply does not apply to the aveira of murder, regardless of how the murder is committed. A person would thus not have the right to passively allow retzicha (and, by extension, the other two aveirot chamurot) to spare his life. Rav Chayim explains the machloket between the Rivam (who allows passive acts of giluy arayot under the threat of death) and the Rambam (who does not) as a machloket about the true nature of the mai chazit logic.
Another question might pertain to a case in which two lives are threatened unless one is taken. Can a person actively kill one person so as to prevent the death of two? If piku'ach nefesh does not apply to murder (meaning, murder cannot be perpetrated to save lives), then even in this instance two lives may not be spared by committing a murder. Alternatively, however, perhaps two lives are truly "redder" than one, and I should therefore allow sparing two lives at the expense of one. Some quote the Me'iri as claiming that indeed two lives should be spared by taking one life - a position which clearly allows piku'ach nefesh to operate in the area of the issur retzicha under certain conditions.