Deuteronomy 19:10-13

10That innocent blood may not be shed in thy land, which the LORD thy God giveth thee for an inheritance, and so blood be upon thee. 11But if any man shall hate his neighbour, and lie in wait for him, and rise up against him, and smite him mortally that he dieth, and he shall flee into one of these cities:1 12Then the elders of his city shall send and bring him from there, and deliver him into the hand of the avenger of blood, that he may die. 13Thy eye shall not pity him, but thou shall remove the guilt of innocent blood from Israel, that it may be well with thee.