מדרש אויף במדבר 35:19

(Bamidbar 35:19) "the avenger, he shall kill the murderer": The mitzvah is the avenger's. Whence is it derived that if he has no avenger, beth-din designates one for him? From "The avenger, he shall kill the murderer when he comes upon him" — in any event.

Why is it stated "redeemer of blood"? Because it says, "If the avenger of blood finds him, he may kill him" (Numbers 35:27), I would think that he may be killed without any formal procedure. Therefore, it is stated "until they stand before the congregation" (Numbers 35:12), which implies that there must be a legal proceeding. If one was pursuing another to kill him, and they said to him, "You are a member of the covenant community," he should know that it is written in the Torah, "Whoever sheds the blood of man" (Genesis 9:6), etc. He may say, "Even so, he is a murderer; let him die before he kills." He may kill him and save the life of this person. Similarly, if one was pursuing a woman to rape her, and they said to him, "She is a married woman," he should know that it is written in the Torah, "The adulterer shall surely be put to death" (Leviticus 20:10), etc. He may say, "Even so, he is an adulterer; let him die before he commits adultery." He may kill him and save the life of this woman by sacrificing his own life. I might think that he may kill him as soon as he pursues him, whether for murder or adultery. Therefore, it is stated, "And the murderer shall not die until they stand before the congregation" (Numbers 35:12). The Sages taught in a Baraita: "And the murderer shall be put to death" (Numbers 35:30) - it is a mitzvah in the hand of the redeemer of blood. If there is no redeemer of blood, he does not have the authority to execute the murderer. These are the words of Rabbi Yossi HaGelili. Rabbi Akiva says: The authority to execute the murderer is in the hand of the redeemer of blood, and all people are obligated concerning him. What is the reason for Rabbi Yossi HaGelili's opinion? Is it not written, "If a man commits murder" (Numbers 35:16)? And Rabbi Akiva, who is it written for? Rav Zutra bar Tovia said that Rav said: It refers to a murderer who went outside the designated boundary and the redeemer of blood found him and killed him. The one who is killed is executed for his own death. This statement does not follow the opinion of Rabbi Yossi HaGelili or the opinion of Rabbi Akiva. This is what Rabbi Eliezer said: "Until they stand before the congregation" - what is the purpose of this phrase? Since it is written, "And the redeemer of blood shall kill the murderer; when he meets him, he shall kill him" (Numbers 35:19), I might think that the redeemer of blood can immediately kill him. Therefore, it is stated "until they stand before the congregation." Rabbi Yossi HaGelili and Rabbi Akiva derive the same interpretation from the phrase "until they stand." It is derived from a teaching: How do we know that the Sanhedrin, when they see a person who has killed a soul and is not liable to be executed until he stands before the court, can still judge him and impose the death penalty? It is derived from the phrase "until they stand before the congregation."

Or with a wooden implement, why is it stated? Because it says, "And if a man strikes his slave or his female slave with a rod," I understand that whether he struck him with a matter that can cause death or with a matter that cannot cause death, it is stated, "Or with a wooden implement," to indicate that he is not liable unless he struck him with a matter that can cause death. And from where do we derive that in a place that can cause death? It is derived from the phrase, "And if a man is quarrelsome, etc.," which indicates that he is not liable unless he struck him with a matter that can cause death. And concerning a place that can cause death, I have only that if he struck him with a wooden implement, he is liable. From where do we derive that throwing boards and wooden beams upon him are also included? It is stated, "He is a murderer; he shall surely be put to death." From where do we derive that if he struck him with a stone, you would say that he struck him with a clump of salt or with a dried fig cake, or he dropped a basket filled with dirt or a basket filled with pebbles upon him? It is derived from the phrase, "And if he struck him with a stone in his hand," or with a wooden implement. From where do we derive that if he struck him on the head of a beam or on the helm of a ship, you would say he is liable? It is stated, "Or with a wooden implement in his hand." From where do we derive that if one pushes his fellow into water or into fire, or incites a snake against him, you would say it is only if he is able to cause his death? It is stated, "In his hand," to include even if he cannot cause his death. As it is said, "The hand of the witnesses shall be first upon him," and if he fled to Babylon, they could go there to execute him. You would say that the redeemer of blood should kill him, etc. From where do we derive that even if he said, "I am unable to accept him," it is stated, "When he strikes him," to include even in a case where he cannot accept him. The redeemer of blood should kill the murderer; it is a mitzvah for the redeemer of blood to do so. And from where do we derive that if he does not have a redeemer of blood, the court appoints a redeemer of blood? It is stated, "When he strikes him." Why is it stated "redeemer of blood"? Because it says, "And if he struck him with an iron implement, or with a stone in his hand," [or with a wooden implement in his hand]. I have only that if he killed him with these, he is liable, but with what did he kill him?