"It is forbidden to murder, as it says "You shall not murder" (Exodus 20:13, Deuteronomy 5:17). A murderer must be put to death, as it says "He shall be avenged" (Exodus 21:20, see Leviticus 24:17,21); it is forbidden to accept compensation from him instead, as it says "You shall not take redemption for the life of a murderer...; and there shall be no atonement for the blood that was spilled... except the blood of him that spilled it" (Numbers 35:31-33). It is forbidden to execute a murderer before he has stood trial, as it says "And the murderer shall not die until he stands before the congregation for judgment" (Numbers 35:12). However, we are commanded to prevent an attempted murder by killing the would-be murderer if necessary, and it is forbidden to refrain from doing so, as it says "And you shall cut off her hand; you shall not be merciful" (Deuteronomy 25:12); and similarly for attempted fornication, as it says "[If the man seizes her and lies with her...] just as a man rises up against his friend and murders him, so is this thing"(Deuteronomy 22:26). It is forbidden to refrain from saving life when it is in one's power to do so, as it says "You shall not stand on your friend's blood"(Leviticus 19:16)."