Deuteronomy 19:10-13

19:10 You must not shed innocent blood in your land that the Lord your God is giving you as an inheritance, for that would make you guilty. 19:11 However, suppose a person hates someone else and stalks him, attacks him, kills him, and then flees to one of these cities. 19:12 The elders of his own city must send for him and remove him from there to deliver him over to the blood avenger to die. 19:13 You must not pity him, but purge out the blood of the innocent from Israel, so that it may go well with you.