According to Rabbi Yoḥanan quoting Rabbi Shimon ben Yehotzadak there are only three mitzvot that are so severe that a person should give up his life rather than perform the forbidden acts. Those mitzvot are:
- avoda zara (idol worship)
- gilui arayot (forbidden sexual relations)
- shefikhut damim (murder)
The Gemara presents Biblical passages as the source for this rule regarding Avoda Zara and Gilui Arayot. When it comes to Shefikhut Damim the Gemara argues that no proof-text is necessary, as it is a sevara – a logical argument – that murder cannot be permitted to save a life. To illustrate the sevara, the Gemara tells of a person who approached Rabba with the following question:
“The ruler of my village came to me and said ‘kill that person, and if you do not then I will kill you.’ Can I follow his order so that I will be able to save myself?”
“Allow yourself to be killed, but you may not kill another. Who says that your blood is redder than his? Perhaps his blood is redder than yours.”
On a simple level, Rabba’s argument is that we cannot tell whose life is more valuable, so we will not allow you to save your life at the expense of another.
Rabbeinu Yehonatan explains Rabba’s answer by arguing that really the laws of the Torah are so important that we would not allow them to be “pushed aside” even at the expense of human life. The reason the halakha permits someone to transgress a serious prohibition – like Shabbat – in order to save a life is because we weigh that single ḥillul (desecration of Shabbat) against the potential Shabbatot that the person will observe in the course of his lifetime. In our case the argument is that we cannot possibly know “whose blood is redder” i.e. who will live longer and perform more mitzvot.