The Chida (Sefer Devash L’fi) writes: One must be very careful not to disparage Jewish people. We see that Moshe Rabenu called the Jewish people “rebels” and he was punished for this. Aviyah spoke badly about the people of Yehuda and said (2 Divrei Hayamim 13): “With you are golden calves”, and it says that Hashem punished him for this. Zechariah said that the people “abandoned Hashem” and he was punished. Yeshaya Hanavi, who was equal to Moshe, was punished and killed by Menashe for saying that the people had “impure lips”.
He continues: Hashem wants us to defend the Jewish people. Chazal say that Gideon was chosen as leader because he spoke well of the people. A person who gives mussar to crowds must be very, very careful not to insult the honor of Klal Yisroel and not to speak badly about them. Instead, he should rebuke them with sweet words.
We see from his words how much one has to be careful not to speak badly of others.