Deuteronomy 19:10-12

that innocent blood be not shed in thy land, which the LORD thy God giveth thee for an inheritance, and so blood be upon thee. But if any man hate his neighbour, and lie in wait for him, and rise up against him, and smite him mortally that he die, and fleeth into one of these cities: then 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.