פּירוש אויף דברים 21:1

כי ימצא חלל, “if someone unidentifiable is found slain,” after the Torah discussed certain aspects of war between nations, it now turns to the result of individuals who had apparently fought one another and the victor had escaped as there had not been any witnesses.

If a corpse is found After mentioning the case of war against our enemies, Scripture now discusses the case when a man wages war against his fellow man, and a corpse is found in the land of Israel, the murderer of which is unknown.

“When a slain person is found on the land, etc.” Rabbi Yitzchak Abarbanel raised a difficulty regarding this section, questioning why it is placed here among the four sections dealing with matters of war. The Rabbi gave a weak reason for this, and other commentators who approached this to explain the connection did so with forced explanations. Now lift up your eyes and see what our Sages said (Sotah 46a): “It was taught: Rabbi Yochanan ben Shaul said: Why did the Torah command to bring the heifer whose neck is broken? The Holy One, Blessed be He, said: ‘Let the heifer which has not produced fruit come and have its neck broken in a place that does not produce fruit, and atone for the one whom they did not allow to produce fruit.’” The Gemara questions: “If this refers to procreation, then would we not perform this ceremony for someone who was castrated or elderly? Rather, it refers to [the inability to perform] commandments.”
And in the previous section it states, For you may eat from it, but you shall not cut it down; only a tree that you know is not a food tree, and our Sages said in the Tractate Ta’anit (7a), “If he is a worthy Torah scholar, from it you may eat and you shall not cut it down, but a tree that you know is not a food tree refers to an unworthy Torah scholar, etc.”
Thus, any intelligent person can see with their mind’s eye that the connection between these two sections is self-evident: If the reason for the heifer whose neck is broken is because they did not allow him to produce fruit through procreation, then it is appropriately connected to the previous section which is particular about not cutting down a fruit-bearing tree. What difference is there between the fruit of man and the fruit of the ground? They share the same law, as it says, For man is a tree of the field. And if [the reason is] that they did not allow him to produce the fruits of commandments, as per the conclusion, behold, in the previous section as well, there is concern not to cut down the Torah scholar who produces fruit — the tree of life according to its kind. I am surprised at all the commentators that such a simple matter was hidden from their eyes.