וַיָּשָׁב רְאוּבֵן אֶל־הַבּוֹר
And Reuben returned to the pit.
Reuben had earlier declared that no blood should be spilled, with the intention of saving Joseph. Strangely, however, during the actual sale of Joseph, Reuben had disappeared. Where was Reuben during Joseph’s sale? R. Eleazar said: He was taken up with his fasting and sackcloth, and when he became free he went and looked into the pit (Midrash Rabbah: Gen 37:19).
The Midrash’s comments refer to an earlier, seemingly unrelated episode. And it came to pass, when Israel lived in that land, that Reuben went and lay with Bilhah his father’s concubine; and Israel heard it. Now the sons of Jacob were twelve. (Bereishis 35:22). The Gemara in Shabbos 55b states: Whoever maintains that Reuben sinned is merely making an error, for it is said, ‘Now the sons of Jacob were twelve,’ teaching that they were all equal. Rashi explains: When Rachel died, Jacob took his bed which was placed regularly in the tent of Rachel…and placed it in the tent of Bilhah. Reuben came and resented the insult to his mother and said, ‘If my mother Leah was subordinate to Rachel, must she also be subordinate to Rachel’s handmaid?’ Therefore, he rearranged Jacob’s bed by moving it to Leah’s tent (Rashi).
Why did Reuben choose this specific occasion to repent for that earlier act? Because only now did Reuben finally realize the staggering implications of his sin. After the brothers witnessed Reuben acting in a disrespectful manner toward their father in the Bilhah incident, their own respect for Jacob declined. While from the perspective of the deed itself Reuben’s infraction was minor, the consequence of the deed was devastating. Reuben undermined Jacob’s authority in his own household, and the ensuing loss of respect for Jacob by Reuben’s brothers ultimately resulted in their proposal to kill Joseph. Reuben finally understood the full implications of his earlier actions, and therefore, immediately after hearing their threat to kill Joseph, he was taken up with his fasting and sackcloth. (Mipeninei Harav, pp. 398-399; Boston undated)