While all of Yosef's brothers were plotting to kill him only one of them stood up and suggested that killing him was not necessarily the best solution. Why was Reuven the only one who felt this way? Why did he suggest that the alternative be throwing him into a pit? Furthermore, why did he tear his clothing when he returned to the empty pit? Why was his reaction to the sale of Yosef so dramatic? Reuven then went to his brothers and said, (37:30) "The boy is no longer there and I, where shall I go?" Why did he assume that the blame for Yosef's disappearance would be put squarely on his shoulders?
Rav Samson Raphael Hirsch offers the following answer. Reuven felt guilty from the outset. After all, wasn't Reuven in direct competition with Yosef for the Berachot? Didn't Reuven have the most to gain from Yosef's "disappearance?" Deep down, Reuven understood his own rivalry with Yosef, yet on the surface he managed to suppress those feelings. When the brothers wanted to kill Yosef, Reuven knew that it was the wrong thing. His conscious mind would not allow him to let it happen, and he instead had Yosef tossed into the pit. Yet once Yosef was in the pit, Reuven let him stay there for a while, perhaps because he subconsciously still felt animosity toward Yosef. When he finally mustered up the strength to return to the pit to save Yosef and found it empty, he blamed himself for not following through earlier and saving Yosef outright.
This point is expounded on by Rashi who says that Reuven had intended to save his brother and return him to his father. One may ask on this statement by Rashi, isn't it evident by Reuven's next statement, "The boy is gone and where can I go"? There Rashi explains it to mean where can I flee from my father's grief?" But from this statement it seems as though he was concerned for his fathers well being and not that of Yosef. On this Rabbi David Feinstein explains that he did not want to take credit for his true motive, therefore he attributed his grief solely for that of his father. He also adds that Reuven reacted this way because no one would believe that this concern for Yosef was his true motive and therefore he needed to express an anterior motive.
We must now try to understand what Reuven really meant when he said (37:21) "let us not kill him." The Or Hachaim states that Reuven meant let us not kill him directly. Rather, let us cause his death in directly. When he says later (37:22) "Do not shed his blood, throw him into the pit," this was his argument to his brothers. He only pretended to agree to think that his death should be caused because he saw this as the only way to save his brother. Proof for this comes from the fact that Reuven returns to the pit presumably with the intention of saving his brother.
The Or Hachaim asks another question on this phrase said by Reuven. What was meant by Reuven's argument when he complained to his brothers and said, "Where shall I go?" Yosef would have been lost even if Reuven's suggestion to keep him in the pit had been carried out. Reuven never told his brothers that he wished to save Yosef and return him to his father. In reality says the Or Hachaim Reuven was arguing that since he was the eldest his father would hold him responsible for finding Yosef. If Yosef would have remained in the pit then Reuven could have at least brought his remains to his father. Now that Yosef was missing though he was no longer able to do this. As a result said the Or Hachaim the brothers decided to dip Yosef's coat into the blood of a dead goat.