Halakhah zu Dewarim 25:11

That is that He commanded us to save someone pursued, even through the life of the pursuer - meaning that we are commanded to kill the pursuer, if we are only able to save the pursued by [taking] the life of the pursuer. And that is His, may He be exalted, saying, "And you shall cut off her hand; you shall have no pity" (Deuteronomy 25:12). And the language of the Sifrei (Sifrei Devarim 292:5-293:1) is, "'By his genitals' (Deuteronomy 25:11) - what is unique about his genitals? They place his life in danger - hence it [comes] with, 'And you shall cut off her hand.' This teaches that you are obligated to save him through [cutting off] her hand. From where [do we know] that if you are not able to rescue him through her hand, that you may rescue him with her life? [Hence] we learn to say, 'you shall have no pity.'" Behold that the content of this command has already become clear to you; and that His saying, "the wife of one," was the verse merely speaking with what is common. But the intention is to save the pursued through [injuring one] of the limbs of the pursuer. And if it is impossible to save him without the death of the pursuer, he should kill him at once. And the laws of this commandment have already been explained in Chapter 5 of Sanhedrin. (See Parashat Ki Tetzei; Mishneh Torah, Murderer and the Preservation of Life 1.)