When the brothers decided to kill Joseph, Reuben stepped in. Targum Pseudo-Jonathan on (Genesis 37:22) makes his motive explicit: because he would deliver him from their hand, and restore him to his father.
Reuben did not argue. He did not plead. He did not appeal to the brothers' sense of mercy — he knew they had none left that day. Instead, he proposed a compromise that looked just as cruel: throw him into this pit in the wilderness, but the hand of the slayer stretch not forth against him. No blood on the brothers' hands. Death by dehydration, technically, not by them.
The sages admired the strategy. Reuben could not stop ten men mid-rage. He could only slow them down. A pit was reversible. A sword was not. If Joseph went into the pit, Reuben could come back later — alone, at night — and pull him out.
But the Targum also tells us, later in the chapter, why Reuben missed the crucial moment. He was fasting. He was sitting on the hills, wrapped in sackcloth, doing teshuvah for the incident with Bilhah (the same story the Targum reinterpreted in chapter 35). The eldest son of Jacob, at the very moment his baby brother needed him, was doing the painful, lonely work of his own repentance.
The Targumist is showing us a tragic irony. Reuben's plan to save Joseph failed because Reuben was busy saving himself. Sometimes the people who love us most miss the moment. Not from indifference. From the cost of their own prior mistakes.