John Calvin Commentary Genesis 37:21

"And Reuben heard it, and delivered him out of their hand, and said, Let us not take his life." — Genesis 37:21 (ASV)

And Reuben heard it. It is good to observe, while others were rushing to shed his blood, by whose care Joseph was preserved. Reuben, doubtless, in one affair, was the most wicked of them all when he defiled his father’s bed; and that unbridled lust, involving other vices, was the sign of a depraved nature. Now suddenly, he alone, showing regard for piety and mindful of his duty as a brother, thwarts the impious conspiracy.

It is uncertain whether he was now seeking a way to make some compensation, so that he might be restored to his father’s favor. Moses declares that it was his intention to restore the boy safely to his father. From this, the conjecture I have stated is probable: that he thought his brother’s life would be a sufficient price by which he might reconcile his father to himself.

Whatever the case, the humanity he showed in attempting to free his brother is proof that he was not abandoned to every kind of wickedness. And perhaps God, by this sign of his repentance, intended to some extent to lessen his former disgrace. From this we are taught that people’s characters are not to be judged by a single act, no matter how atrocious, to the point of causing us to despair of their salvation.