Genesis 42:37 - Ellicott's Commentary On The Whole Bible

Slay my two sons. — Reuben does not suppose that Jacob would really put his grandchildren to death. but simply means to offer his father a strong assurance that Benjamin would run no danger. He regarded the risk as so slight that he was willing to stake the lives of two of his children, perhaps all he then had, upon Benjamin’s safe return. To take such a proposal as meant literally is irrational. But it was but feeble talk, in agreement with the general weakness of Reuben’s character.