Due to current events, I would like to discuss the halachos of a killing a rodef. The Rambam (Hilchos Rotzeiach 1:6) writes that if a person is chasing after another to kill him, all of Klal Yisrael is obligated to save the one being chased, even if it requires killing the chaser (the rodef). In halacha 15, the Rambam writes that one who has the ability to save another person from a rodef and does not transgresses a mitzvas assei and two lo saassei. And these aveiros are very serious, for one who squanders the life of a Jew is comparable to having destroyed the entire world, and one who saves the life of another Jew is comparable to having saved the entire world!
Rav Chaim Soloveitchik, zt”l, in his sefer on the Rambam, explains that in order to kill a rodef two factors must be present. First, there must be a necessity to save the life of the one being chased. This is a unique halacha similar to that of pikuach nefesh (but not the regular halacha that anything is permitted for the sake of pikuach nefesh). Second, when one is chasing another person the Torah imposes a punishment that he may be killed. In order to kill a Jew who is trying to kill another Jew these two requirements must be met: the life of the chased will be saved and the chaser can be held responsible.
One example where the second requirement is not met is a case in which the chaser is not morally responsible; rather, nature is to blame. For example, if a baby being born has already stuck its head out of its mother, we cannot kill it, even though it is threatening the life of its mother, because it is not the will of the baby to kill its mother. Nature is simply taking its course. If, however, the head of the baby has not yet emerged, it is permitted to kill the baby. Even though the baby has no intent to kill and nature is simply taking its course, an unborn baby is considered less of a nefesh than a regular person. Therefore, it is sufficient that the first requirement is met – that the life of the chased will be saved.
The halacha is that if a person can prevent the chaser from killing the victim without killing the rodef he must. If he has this opportunity but decides to kill him anyway, he is a murderer, writes the Rambam. Nonetheless, the Rambam writes, beis din should not kill him for this aveirah.
There are many explanations for this halacha. The Brisker Rav suggests that since everyone has an obligation to kill the rodef and since another person may not have the opportunity to stop him by other means, the rodef’s life is considered forfeit and thus anyone who kills him – even someone who could stop him some other way – is not liable for his death.
There is another halacha relevant to this topic. If a person is being attacked by a murderer, he has the right to defend himself and kill his attacker, even if – write many poskim – he can stop his attacker without killing him. He need not, for example, shoot him in the leg if he has a gun. It is only bystanders who must use other means of stopping an attack if they are available to them. The person being attacked himself need not resort to other options.
One more point that relates to recent events: One is not liable for murder if one kills a person who was already shot and lying on the floor. This person is considered a treifah, and one is not liable for killing a treifah.