Deuteronomy 19:11-13

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.