What were the people accusing Moshe of?
Ramban#1 (citing Targum Onkelos), Rashbam and Seforno: Regarding the two hundred and fifty men, 1 they argued that Moshe and Aharon should have chosen some other area of Korbanos (Rashbam) 2 - which would have allowed a number of Kohanim to perform simultaneously - with which to test them, and not the Ketores, 3 where only the Kohen who brought the Olas Tamid was permitted to bring.
Ramban #2 (citing Ibn Ezra): The tests that Moshe made did not prove that the tribe of Levi and Aharon were chosen 4
Rashbam: They conceded that Dasan and Aviram deserved what they got.
Ramban: Using the staff or some other method.
Ramban: Which Hashem had not commanded them, and which they chose of their own accord.
Ramban (according to the Ibn Ezra): They did not believe the outcome of the test of Ketores at all, as is evident from the test of the sticks later, on Pasuk 17.
Ramban: They did not question the punishment of Dasan and Aviram, which was by Divine Decree, as hinted in the previous chapter, Pasuk 24.
Ramban: Since they had seen how fire had emerged from before Hashem and had consumed the Korbanos that he brought.
Ramban: So that all the tribes have a share in the Avodah.
Ramban: And which is why, in Pasuk 23, Aharon's stock sprouted blossoms on behalf of the tribe of Levi.