Sefer HaChinuch ספר החינוך

To not have concern about the pursuer: That we have been prevented from having compassion for the life of the pursuer. And the elucidation of this commandment is like I have written in its positive commandment in this Order (Sefer HaChinukh 600): That the tradition came to us about two sins — which are murder and sexual prohibitions — that if we see him pursuing to do one of them, we must always prevent him with all of our strength. And if he does not want to desist with words from doing the sin, and we are able to save from his hand the man or woman pursued, with one of his limbs, we must chop it off. And if it is impossible for us to save the pursued if we do not kill the pursuer, we must kill him. And about this comes the prevention to us, that we should not have compassion upon him; but rather we should kill him nonetheless — if it is impossible for us to save the pursued in any other way than by [taking] his life. And about this is it stated (Deuteronomy 25:12), “You shall cut off her hand; show no pity.” And the language of Sifrei is “‘You shall cut off her hand’ teaches that you are obligated to save him with her hand. And from where [do I know even] with his life? [Hence,] it teaches us to say, ‘show no pity.’”

And the roots of the commandment and its content is elucidated — as is the custom of the book — in its positive commandment (Sefer HaChinukh 600) in this Order.