Ten brothers stood before him, and Joseph picked Simeon. Targum Pseudo-Jonathan on Genesis 42:24 preserves the reason the Torah leaves quiet: Simeon "had counselled them to kill him."
The brother who had pushed for murder
At the pit in Dothan, Genesis 37:20 records the brothers' plan: "Let us slay him, and cast him into some pit." The Torah does not name which brother was the loudest voice. The Aramaic paraphrase, which took its final form in the Land of Israel around the seventh or eighth century CE, does: Simeon. This is consistent with the broader portrait of Simeon in the Torah — he and Levi massacred the men of Shechem after Dinah's violation (Genesis 34:25), and Jacob on his deathbed would later rebuke both of them for their "instruments of cruelty" (Genesis 49:5-7). Simeon was the family's sharpest edge.
Joseph withdraws to weep first
The Targum also preserves a small, tender note: Joseph "withdrew from them and wept, and returned and spake with them." Before binding Simeon, he cries. The rabbinic tradition reads this pause as evidence that Joseph's severity was always layered over love. He chose the right brother to detain, but he wept before doing it. Justice and grief can coexist.
The takeaway
Joseph's choice is precise. He did not pick the youngest, or the weakest, or the easiest. He picked the one whose voice had pushed the crime forward. And before he acted, he stepped out of the room and cried.