Yoma

Rut whence do we know that this principle applies in the case of a murder? — This is reasonable. For there was a man who came before Raba and said to him: The lord of my village told me: Kill so-and-so, and if you will not, I shall kill you! — He [Raba] answered: Let him kill you, but do not kill! What makes you see that your blood is redder than his? Perhaps the blood of that man is redder than yours?1 There was a woman with child who had smelt [a dish]. People came before Rabbi [questioning him what should be done]. He said to them: Go and whisper to her that it is the Day of Atonement. They whispered to her and she accepted the whispered suggestion, whereupon he [Rabbi] cited about her the verse: Before I formed thee in the belly I knew thee.2 From her came forth R. Johanan. [Again] there was a woman with child who smelt [a dish]. The people came to R. Hanina, who said to them: Whisper to her [that it is the Day of Atonement]. She did not accept the whispered suggestion. He cited with regard to her: neighbour's life is as clear to Him as your own; and no blood is ‘redder than the other’. Hence you must not kill someone else, in order to save life. But if someone comes to murder you, self-defence is not only permissible, but mandatory.