"The Plague Had Begun..." (17:12)

While the plague followed after complaints of the Jewish people (v. 6), this cannot have been its sole cause, since we find that the Jewish people complained on numerous occasions and were not punished with a plague. Rather, the plague was clearly a punishment for Korach's rebellion that was catalyzed by the complaints of the Jewish people later on.

Even though Korach incited an entire congregation against Moshe, Nevertheless the people were not punished by the plague until they actually rebelled. From this we can learn a powerful lesson concerning our attitude to others: Even if a person is lacking much in his personal level of observance, so long as he has not actively rebelled against God, one can be sure that he still finds grace in the eyes of the Almighty.

(Based on Likutei Sichos vol. 28, p. 2, note 14; ibid. p. 6)