Genesis 4:1-8 goes as follows:

Now the man knew his wife Eve, and she conceived and bore Cain, saying, “I have gained a male child with the help of the L-RD. She then bore his brother Abel. Abel became a keeper of sheep, and Cain became a tiller of the soil. In the course of time, Cain brought an offering to the L-RD from the fruit of the soil, and Abel, for his part, brought the choicest of the firstlings of his flock. The L-RD paid heed to Abel and his offering, but to Cain and his offering he paid no heed. Cain was much distressed and his face fell. And the L-RD said to Cain,

Why are you distressed,

And why is your face fallen?

Surely, if you do right,

There is uplift.

But if you do not do right,

Sin crouches at the door;

Its urge is toward you,

Yet you can be its master.

Cain said to his brother Abel….and when they were in the field, Cain set upon his brother Abel and killed him.

The question that has bothered all readers of this passage has been the following: what did Cain say to Abel? The Samaritan Pentateuch, the Greek Targum Shiv‘im (the Septuagint), and the Aramaic Targum all add the words that translate into “Come, let us go into the field.” Thus, according to those versions, we know what Cain said. Rashi, Abraham ibn Ezra, and Ramban, each in his own way, “fill in” the gap as well.

But the Masoretic text of Genesis 4:8has nothing of this conversation. And, of course, as Jews committed to the Masorah, we must try to understand the text as it is. What, then, is the understanding of the narrative according to our Masorah? Must we “fill in” something along the lines of the Targum as well? Is the point only that the Torah omitted words that are obvious and can be gleaned from the content?

My colleague R. Ya‘akov Neuberger once suggested the following explanation: The fact that the Torah does not record what Cain said to Abel precisely reflects the fact that indeed, he said nothing to him. And that is the whole point of the story. When communication between people breaks down, when people stop talking to each other, anything could result, even, God forbid, murder. (Rav Neuberger’s idea was cited in Douglas B. Sagal, “‘Imaginative Insight:’ Midrash and African-American Preaching,” Judaism 50:1 (Winter 2001), pp. 3-16, on p.7.)

What was the original dispute about? The Midrash Bereshit Rabbah (xxii:7) suggests three possibilities.

And Cain Spoke unto Abel his brother.

About what did they quarrel?

“Come,” said they, “let us divide the world.” One took the land and the other took the movables. The former said, “The land you stand on is mine,” while the latter retorted, “What you are wearing is mine….”
R. Joshua of Sikhnin said in R. Levi’s name: “Both took land and both took movables, but about what did they quarrel? One said, ‘The Temple must be built in my area,’ while the other claimed, ‘It must be built in mine.’”
Judah b. Rabbi said, “Their quarrel was about the first Eve.” Said R. Aibu: “The first Eve had returned to dust.” Then about what was their quarrel? Said R. Huna, “An additional twin was born with Abel and each claimed her. The one claimed: ‘I will have her, because I am the first born,’ while the other maintained, ‘I must have her because she was born with me.’”
It is appealing to suggest that this Midrash is portraying not just the specific dispute between Cain and Abel but from a universal perspective, presenting three reasons why wars break out. One reason is an economic one. A second reason concerns issues of power. (Like the Ramban on the parashah of eshet yefat to’ar [Deuteronomy 21:10-14], this Midrash might understand issues concerning sexual violations as essentially revolving around issues of abuse of power.) A third reason is the religious one. Who will have the right to claim the Temple?

After committing the horrible crime, Cain cries out (verse 13) “My sin is greater than I can bear.” Ramban interprets these words as a confession.

Cain said, “It is true that my sin is too great to be forgiven…” Ramban writes:

The sense is that Cain said before God, “Behold, my sin is great, and you have punished me exceedingly, but guard me that I should not be punished more than You have decreed upon me, for by being a fugitive and wanderer, and unable to build myself a house and fences at any place, the beasts will kill me, for Your shadow has departed from me.” Thus, Cain confessed that man is impotent to save himself by his own strength but only by the watchfulness of the Supreme One upon him (Ramban: Commentary on the Torah: Genesis, translated by Rabbi Dr. Charles Chavel, p. 91.)

Interestingly, although all his other descendants perished even after Cain’s confession (and, presumably, repentance), according to one opinion in Hazal, a righteous descendant of Cain did emerge. This is in accordance with the view of Bereshit Rabbah (23:4), that Na‘ama (the sister of Tubal-Cain {Genesis 4:22}) was Noah’s wife. As Ramban (ad loc.) remarks, citing the Midrash:

“And why did they call her Na ‘amah (which means lovely)? Because her deeds were lovely and pleasant.” By this the Rabbis meant to say that she was famous in those generations because she was a righteous woman and she gave birth to righteous children. That is why Scripture mentioned her. If so, a small remembrance of Cain was left in this world (Ramban, ibid., p. 94).