The Chofetz Chaim explains that the antidote to lashon hara is “loving my neighbor.” If I, in fact, viewed him as connected to me, I would never speak negatively about him. It would be like badmouthing myself.
This seems to be the answer to this Rashi. The Jewish nation is one. If such an incident of vicious slander could occur, it reflected on the state of nation. If the people had been on a higher level, this could not have transpired. It meant the nation as a whole was lacking in a key ingredient – a sense of common destiny, a sense of brotherhood, the sense that I am one with my fellow Jew. And that is why the nation deserved to be punished. As children of Avraham, Yitzchak, and Yaakov, we share a common heritage and destiny. We are bound together for eternity. We are one.
For that reason, when Moshe witnessed this act of cruel gossip mongering, he took it as a sign of the health of the nation. If the bottom has sunk this low, the head can’t be that much higher. He then understood why it is that the Jews deserved such treatment. If any members of other nations degrade one another, there isn’t much fault found with them. If a member of Klal Yisrael speaks badly about another, that bodes serious consequences. We are held to a higher standard.
This concept is a powerful lesson to us about the unity of the Jewish people, our common destiny, and the power of each individual to impact the whole.