Le Deutéronome 21:5 Midrash : Devarim Rabbah, Sifra, Midrash Tanchuma & Sifrei Devarim

“Judges and officers you shall place for you within all your gates that the Lord your God is giving you for your tribes, and they shall judge the people with fair judgment” (Deuteronomy 16:18).
“Judges and officers you shall place for you within all your gates that the Lord your God is giving you for your tribes, and they shall judge the people with fair judgment” – halakha: A person’s relative, what is the ruling, is it permitted for him to sit in judgment for him? This is what the Sages taught: These are considered relatives:¹Whose testimony is not accepted. One’s father, one’s brother, one’s father’s brothers, and one’s mother’s brothers…Why is it so? Just as a relative is ineligible to testify, so is he ineligible to judge. What do you see that leads you to say so? Rabbi Shimon ben Yoḥai said: It is written: “The priests, sons of Levi, shall approach, as the Lord your God chose them to serve Him, and to bless in the name of the Lord, and by their word shall be every dispute and every case of leprosy” (Deuteronomy 21:5). Come and see that it compares leprosy to disputes, and disputes to leprosy. Just as [the examination of] leprosy is only during the day, so judgment is only during the day, and just as disputes exclude relatives, so, too [the examination of] leprosy excludes relatives.
Rabban Shimon ben Gamliel said: Do not regard justice lightly, as it is one of the stanchions that support the world. Why? It is as the Sages taught: The world stands upon three matters: Upon justice, upon truth, and upon peace. Consider that if you distort judgment you destabilize the world, as it is one of its stanchions. The Rabbis say: The potency of justice is considerable, as it is one of the legs of the Divine Throne. From where is this derived? “Righteousness and justice are at the base of Your throne; kindness and truth greet Your countenance” (Psalms 89:15). The Holy One blessed be He said: ‘Since that is so, the punishment for injustice is harsh; be careful.’ From where is this derived? It is from what we read in our context, “Judges and officers.”

9) If in the end all Jews are included (for inspection), why is it written "of his sons, the Cohanim"? To teach that the pronouncement of "tumah" and "taharah" must be made by a Cohein. How is this effected? An Israelite sage inspects the plague-spots and says to the Cohein, even if he himself is unversed (in their inspection): Say "tamei," and he says "tamei"; say "tahor," and he says "tahor." Variantly: What is the intent of "or to one of his sons, the Cohanim"? Because it is written (Devarim 21:5): "And by their word (that of the Cohanim) shall every contention and every plague-spot be (ruled upon)" — Contentions are hereby compared to plague-spots. Just as plague-spots are adjudicated in the daytime, so, contentions are to be adjudicated in the daytime.

And when you approach the battle, the priest shall come forward and address the people (Deuteronomy 20:2): And so [too] does it state (Deuteronomy 21:5) "The priests, the Sons of Levi, shall come forward." And [regarding] anyone who serves the king, when the king makes a decree, he carries out his decree. And the Levites serve the Holy One, blessed be He, and carry out His decrees. "Be strong and resolute, be not in fear or in dread in front of them" (Deuteronomy 31:6). And why? "For the Lord your God Himself marches with you; He will not fail you or forsake you." He does not leave you, and He goes forth in front of you and makes war. And so Deborah says to Barak, "Rise, this is the day on which the Lord will deliver Sisera into your hands; is not the Lord going forth before you" (Judges 4:14). But Barak said to her (Judges 4:8), "If you go with me, I shall go." [So] she said to him, "Do you need me, 'is not the Lord going forth before you?'" [In the case of] a king of flesh and blood, his servants and his troops go forth to war and he is behind them. But the Holy One, blessed be He, is not like this, but rather He is first. And so is it written with David (II Samuel 5:23-24), "Do not go up, etc. but confront them at the baca trees. And when you hear the sound of marching in the tops of the baca trees, then go into action." As they are full of thorns. And why did the Holy One, blessed be He, reveal himself from the tops of the baca trees? Because Israel was in distress. Rabbi Berakhia said, "This is that which is stated, 'I am with him in distress'" (Psalms 91:15). "For the Lord will be going in front of you to attack within the Philistine camp." Hence, "For the Lord your God Himself marches with you." And in the world to come, "Then the Lord will come forth and make war on those nations as He is wont to make war on a day of battle" (Zechariah 14:3). So did Rabbi Tanchuma bar Abba expound.