The 293rd prohibition is that we are forbidden from sparing the life of a rodef.

The explanation of this: we said in the previous mitzvah1 that witnesses may not kill someone who has performed a transgression until the High Court has sentenced him to death; but this applies only if he has already performed and completed the transgression that carries the death penalty. However, when he is still involved in trying to perform the act, he is termed a rodef, and we are then obligated to prevent him from doing the sin he has in mind. If he refuses and persists, we must attack him. If we can stop him by [merely] depriving him of use of a limb, such as cutting off his hand or foot, or blinding his eye, that is fine. But if the only way to restrain him is by killing him, he must be killed before he performs the act. In this case, there is a prohibition to have pity on the pursuer by refraining from killing him.

The source of this prohibition is G‑d's statement,2 "You must cut off her hand [if necessary, to save her victim] and not have any pity."

In the words of the Sifri: "The phrase 'You must cut off her hand [if necessary]' teaches that you must save him [even] by cutting off her hand. What is the source of the law that if you cannot save him only through cutting off her hand, then you must kill her in order to save him? From the phrase, 'and not have any pity.' " There [in the Sifri] it also says: "[The reason the Torah uses the example of] 'his private parts' is because [an attack to] his private parts could endanger his life. In this case 'you must cut off her hand.' So too in any case where his life is in danger, 'you must cut off her hand.' "

Our previous statement that the rodef must be killed does not apply to all cases where a person is attempting to do a transgression. It applies only when one is chasing after another trying to kill him, even should [the rodef3] be a child; or trying to commit rape in a case the Torah terms gilui ervah,4 which obviously includes [to rape] another man. [That the law of rodef applies also to rape is derived from] G‑d's statement5 (exalted be He), "Even if the betrothed girl has screamed out, there would have been no one to come and save her." This implies that would there have been someone to save her, that he would do so with any means at his command. The Torah compares the law of an attempted rape of a betrothed girl and an attempted murder in the verse,6 "This is no different from the case where a man rises up against his neighbor and murders him."

The details of this mitzvah are explained in the 8th chapter of tractate Sanhedrin.