There is a tremendous mussar to be learned, says the Sifsei Tzadik, from the Shevatim's admission of, "Asheimim anachnu...," "We are guilty... [of selling Yosef]," when faced with the accusation of being spies. How do most of us (myself included) usually respond when things don't go our way? We yell that life's unfair. We wonder why G-d has it in for us. We ask why we have it so bad and the other guy has it all. The Shevatim were tzadikim; they deliberated over every detail and possible repercussion of their actions. Surely they had a right to clamor that life's unfair if, despite all their tzidkus, they suffer! Yet, rather than blame G-d or blame fate or wonder why they suffer more than others, their response to adversity was to look at their own shortcomings and blame only themselves.
Following the brothers' admission of guilt, the Torah tells us that Reuvain spoke up. "Vaya'an Reuvain osam leimor, 'Halo amarti aleichem al techetu bayeled....'" (42:22) Reuvain spoke, saying to them, "Didn't I tell you not to harm the child [Yosef]..." Just what everyone needs when they finally admit that they messed up -- someone to jump in with an 'I told you so!' What is Reuvain doing? How can he callously rub salt in the brothers' wounds at this crucial moment?
The Ksav Sofer explains that the Torah here is telling us a "din" in hilchos teshuvah. It's far worse to do something wrong after you were warned not to do it than to do something wrong without being advised or aware of the consequences. That's why a person can only be chayav misa or malkos if they were given has'ra'ah, a warning of what the penalty would be. Reuvain was telling his brothers that if they truly wanted to do teshuvah, they would have to admit not only to doing wrong, but to doing wrong even after being warned of the consequences. Hence the double language, "Vaya'an... leimor," in the pasuk: Reuvain said to the brothers that to do teshuvah properly they must say and acknowledge that they ignored his warning.
It's still a hard pasuk to digest, so let me give you a little Ishbitz torah to make it go down easier. The Beis Ya'akov writes that had the brothers really been convinced that Yosef deserved to die, Hashem would have gone along with their psak. The Rishonim write based on the pasuk, "Elokim nitzav b'adas K-l," that the Sanhedrin has Hashem's backing in every decision they make (see Ramban on the parsha of eidim zomimim, Derashos haRan 11); kal v'chomer Hashem would support the psak of a Beis Din of the Shevatim, even if it meant Klal Yisrael would have one less sheivet. But truth be told, the brothers were not really convinced of their own judgment. No sooner had they paskened than they began to have doubts and hedge their bets. Reuvain had rachmaus and wanted to save Yosef; Yehudah had rachmanus and said to sell, not kill Yosef; none of the other brothers spoke up in opposition and insisted on sticking with the original death penalty. Hashem didn't need to prove the brothers wrong, as in their heart of hearts, the rachmanus they felt for Yosef already told them that they were wrong. Hashem will go along with whatever Beis Din feels is emes, but Hashem will not go along with the plan when everyone really knows it's sheker from the get go.
When the brothers said, "Asheimim anachnu...., " "We are guilty...," they finally acknowledged the rachmanus they felt for Yosef and had been trying to sublimate and ignore for years; they finally admitted that the doubts they felt from the get-go proved that their plan was not just. And who had the most guilt to confess? Reuvain, as it was Reuvain who most felt in his heart that they should have mercy on Yosef; it was Reuvain who begged them from the get-go not to harm Yosef; it was Reuvain who was most aware from the outset that the plan being hatched was a big mistake. Therefore, it is Reuvain who follows the brothers admission of "Asheimim anachnu...," with his own admission of added guilt. When he says, "Didn't I say not to harm the child...?" he is not criticizing his brothers. Rather he is criticizing himself -- if only I had paid more attention to those feelings of mercy and followed through with the rescue of Yosef, how different the situation would be.
What a world of difference the Ishbitzer's perspective makes. Rather than foisting more guilt on his brothers, Reuvain was accepting greater personal responsibility for the events that transpired.