What are the implications of "Lo Hashem Shelachani!"?
Rashi #1: If Korach, Dasan and Aviram will die a natural death, everything that he had done was of his own accord and Korach's claims against him were justified.
Rashi #2 (in Seifer ha'Pardes): It they would die a natural death, then even he would deny his own claims and concede that Hashem did not send him to teach Torah and Mitzvos.
Rashi #3: Moshe was speaking with Ru'ach ha'Kodesh and he knew that they would not die a natural death. 1 What he therefore meant was that, if Hashem did not produce this creation to verify his prophecy, 2 Korach and his congregation would be able to justify their claims that all of the nation are holy ... and that we are raising ourselves above the congregation.
Ramban: He meant that Hashem did not send him to take Yisrael out of Egypt. 3
Rashbam: If Korach, Dasan and Aviram will die a natural death, then Hashem did not send him and the choice to bring Ketores was his own.
Why did Moshe decree this death on Dasan and Aviram more than on the whole of Yisrael, who queried him "Lamah He'elisany mi'Mitzrayim?" on a number of occasions?
Ramban: Because by stating 1 "Ki Sistarer aleinu Gam Histarer!," 2 they committed two grave sins; a. They dishonored their Rebbe's Kavod, and b. they denied all that Hashem had done for them in Egypt, in the desert and Ma'amad Har Sinai, where Hashem had explicitly stated "ve'Gam b'cha Ya'aminu le'Olam!". 3
In Pasuk 13.
Ramban: Korach too, said "u'Madu'a Tisnas'u " (in Pasuk 3). Consequently, he too, suffered the same fate as Dasan and Aviram.
Ramban: And they were now claiming that Moshe was not worthy to lead them and that everything that he had performed with them turned out bad. And it is for the same reason that Moshe was angry with them (See Pasuk 15).