Deuteronomy 19:10-13

10that innocent blood be not shed in the midst of thy land which Jehovah thy 1God giveth thee for an inheritance, and blood come not upon thee.

11But if a man hate his neighbour, and lie in wait for him, and rise up against him, and smite him mortally that he die, and he flee into one of these cities,

12then the elders of his city shall send and fetch him thence, and deliver him into the hand of the avenger of blood, that he may die.

13Thine eye shall not spare him; and thou shalt put away innocent blood from Israel, that it may be well with thee.