Refer to several of the mefarshim (commentaries):
- Rashi - A person who kill his 'brother' by accident, Hashem requires their atonement by the murderer fleeing to one of the Cities of Refuge. And in a case when there are no witnesses G-d will create a Divine atonement.
מיד איש אחיו AT THE HAND OF EVERY MAN’S BROTHER — At the hand of a man who loves him like a brother and slays him accidentally will “I” require it, unless he go into banishment (in one of the Cities of Refuge) and pray for forgiveness for his iniquity. For even one who kills another by accident needs atonement: therefore if there are no witnesses to the deed to make him liable to banishment and he does not humble himself, the Holy One, blessed be He, will require it of him, just as our Rabbis explain the text, (Exodus 21:13) ‘‘But God causes it to come to hand”, in Treatise Makkot (Makkot 10b): The Holy One, blessed be He, causes them (the man who killed by accident and had not expiated the murder and the man who killed with premeditation) to meet at the same inn. The former in ascending a ladder falls upon the latter and kills him and has therefore — the accident having been seen by men — to go to banishment.
- Ibn Ezra - Hashem will avenge the death of man
AND AT THE HAND OF MAN. If many people kill one person or if one man kills another person (even at the hand of every man’s brother), I will avenge that death.
- Sforno - Similarly notes that Hashem will exact retribution from any human that kills another human.
This is the meaning of the apparently redundant words מיד איש אחיו אדרוש את נפש האדם, where the word אדרוש refers to the life of a human being that has already been killed. We find the word אדרוש in both senses depending on circumstances, compare Ezekiel 34,10. The emphasis on איש את אחיו, refers to G’d exacting such retribution from humans for killing humans, seeing that this is not their nature, or should not be their nature. He will not exact retribution from an animal which killed a human being who, in G’d’s eyes, had already forfeited its right to live.
Bechor Shor - Hashem will seek reckoning from both the hand of a man who kills and likewise that of his brother.
Chizkuni - Hashem demands an accounting for anyone that kills themselves or another person, and in the case of straight out murder, G-d demands that the case be settled by tribunal through human court (beis din).
Ha'amek Davar - Hashem as it were is explaining here specifically when a person should be punished. It is different to a time of war and a time of hatred, as then there is a time to kill the perpetrator and there is no resulting punishment. (Like it says in Shevuos 35b that a monarchy that kills one of every six individuals in the world is not punished , and even a Jewish king may wage war even if many other Jews will be killed.)
Rabbeinu Bachya - Similar to Chizkuni, if a man kills and there are witnesses there should be human intervention through the courts and if no witnesses then Hashem Himself will serve as Judge.
The Torah continues ומיד האדם, “and from the human being, etc.” This refers to instances where there are witnesses to the murder. In other words: man must judge murder when there are witnesses, whereas G-d will judge the murderer when there is no evidence which is admissible in a human tribunal.