The Chafetz Chaim was going round from city to city selling his books. One day he arrived in Kelm. The famous Rosh Yeshiva and Baal Mussar, Rav Simcha Zissel Ziv known as the Alter from Kelm, asked him to give a lecture in his Yeshiva, known as the Talmud Torah of Kelm.
The Chafetz Chaim said the following. Moshe Rabbeinu killed the Egyptian. The next day when he rebuked Dassan and Aviram they replied to him, are you going to kill us like you killed the Egyptian? Moshe Rabbeinu was scared and said "Achein noda hadavar", that the news was out what he had done. Rashi explains that he was afraid because he saw Jews speaking Lashon Hara and said that maybe they are not worthy to be redeemed. The Midrash adds that Moshe Rabbeinu said "now I have answer to my question why the Jews are going through such suffering!"
The Chafetz Chaim asked that we know the Jews nearly reached the lowest level of Tumah, they were worshiping idols just like the Egyptians. But that didn't answer Moshe's question why the Jews were suffering. Why was Lashon Hara such a terrible sin more than anything else that caused them to suffer so much?
The Chafetz Chaim gave the following answer. We know that when we do a Mitzva, our good deed creates a good Angel - Malach. A transgression creates a bad Angel. However from a bad action, the Angel created is dumb, and isn't able to speak against the sinner. However when a person speaks Lashon Hara, since the transgression was done through talking, the Angel created can also talk. Not only does that Angel speak about the transgression of Lashon Hara the person who created him spoke, but it also tells over to the Heavenly Court about all the other sins the person has done.
That is what Moshe Rabbeinu meant. Until now the Jews may have sinned but the bad Angel couldn't speak before the Heavenly Court and prosecute the Jews. However, now that I see that they speak Lashon Hara, they have created Angel that can talk and mention all their sins. That is why we must be so careful and refrain from speaking Lashon Hara.
