Deuteronomy 19:10-13

10You 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.

11However, suppose a person hates someone else and stalks him, attacks him, kills him, and then flees to one of these cities.

12The elders of his own city must send for him and remove him from there to deliver him over to the blood avenger to die.

13You must not pity him, but purge out the blood of the innocent from Israel, so that it may go well with you.