(Genesis 4:8) contains one of the strangest silences in the Torah. It says "Cain spoke to Abel his brother," and then nothing. The sentence just stops. The next thing that happens is the murder. What did Cain say? The Hebrew does not tell us. The Targum Jonathan fills that silence with an extraordinary theological debate.
Cain argued that the world was created in goodness but is not governed by justice. God plays favorites. That is why Abel's offering was accepted and his was not. Abel countered that the world is governed according to good works, and there is no favoritism in judgment. His offering was accepted because his deeds were better. Cain then escalated. "There is neither judgment nor Judge, nor another world. No reward for the righteous. No vengeance on the wicked." Abel responded with the opposite creed. "There is a judgment, and there is a Judge, and there is another world." Because of these words, the Targum says, they fought, and Cain "drave a stone into his forehead, and killed him."
The murder weapon is another Targum addition. Genesis does not say how Cain killed Abel. The Targum specifies a stone to the forehead.
But the chapter's strangest claim comes at its very beginning. The Targum says Eve "had desired the Angel" and that when Cain was born, she declared "I have acquired a man, the Angel of the Lord." This reflects an ancient tradition, also found in Pirke DeRabbi Eliezer, that Cain's true father was not Adam but Sammael, the angelic figure. Abel, by contrast, was born "from her husband Adam," as his twin. The brothers were half-brothers at best, one human and one partially angelic, which reframes the entire rivalry.
Cain's offering also gets a specific date. It happened "on the fourteenth of Nisan," the date that would later become Passover. And when God marked Cain to protect him, the Targum says He "sealed upon the face of Cain the mark of the Name great and honourable," the divine Name itself engraved on a murderer's face.