The people were ke-misonenim/like complainers, which was evil in Hashem’s ears.
Misonenim, says Rashi, means seeking a pretext for removing themselves from following Hashem. In other words, there was nothing of substance behind their complaint. The purpose of the “complaint” was to create confusion and chaos, to cover the evil intentions that were lurking behind their words. There was no validity to the complaint; it was pure pretext, similar to the notorious Blood Libel, whose organizers understood that there was not a shred of truth behind the claim. They invented it only to disguise their murderous hatred.
Note that the pasuk says that they were ke-misonenim/like complainers. Had they been actual misonenim, their crime would have been far worse!
The emptiness of their claim should be apparent. They were well-fed by the monn. Ample flocks and cattle accompanied them that easily could have provided extra food for them. Moreover, the magical life that Hashem created for them – the exodus from Egypt, matan Torah, and their imminent entry into a wonderful Land – should have so filled their lives, that they should not have felt that anything could be lacking in it! The only way to understand their behavior is that their complaint was pure pretext. They wanted more distance from Hashem.
Let me relate a similar story. When I was young, I learned in the famed Talmud Torah in Kelm. I was in the company of select, accomplished avreichim. One day, a fire broke out in some of the nearby houses. The Talmud Torah was spared. I remember that one of the stars of the Kollel admitted to me at the time, that while watching the conflagration, a thought had crossed his mind: “It would be a good thing if the Talmud Torah burnt down as well.” He missed his wife and children. He was too embarrassed to return home for a visit. His wife would surely ask him why he left his learning in the middle of a zman. The approaching flames afforded him cover for his secret desire to leave the Kollel to be with his family. To fulfill his wish, he was prepared – on some deep, dark level – to tolerate the loss of the entire Torah institution!
He was an otherwise outstanding person! We see from his example what murkiness exists within the deeper recesses of a person. When a person is unaware of his inner complexity, his thinking is vulnerable to being pulled in different directions without his understanding why. He will complain about the food he remembers tasting as a slave in Egypt, without realizing that his real desire it to run away from the Shechinah.
Self-knowledge, then, is the only way out of the wilderness.