During times when the Torah is the law of the land, if a Jew commits accidental murder against another Jew, he must flee to a Ir Miklat (City of Refuge) or else be subject to revenge killing by the Goel HaDam (Blood Avenger).
Is the Goel HaDam obligated to make a reasonable effort to kill the accidental murderer before he arrives in (or if he leaves) the Ir Miklat, or it is he simply allowed to do so if he wishes? For example, would it be a sin for him to forgive the killer or to decide not to attack despite the opportunity? Is it a mitzvah for the Goel HaDam to complete the act of revenge and/or make reasonable efforts to do so?