הָאִישׁ אֶחָד יֶחֱטָא וְעַל כָּל הָעֵדָה תִּקְצֹף
if one man sins, shall You be angry with the whole congregation?
My grandfather asks, how could they have questions G-d's judgment? Do we not accept that His is the ultimate justice even if we fail to understand it? He refers to Ramban on verse 21 who asked a similar question, "and this is the manner of those who plead for mercy -- to lighten the sin from the whole nation in attributing it to the individual who is guilty for causing it."
My grandfather continues to say, that certainly it did not occur to Moshe and Aharon to cast aspersions on the justice and decress of Hakodehs Baruch Hu. Rather, their approach is similar to Avraham's argument when pleading for mercy for Sdom in Bereishis 18: 25
As Rashi, based on Midrash Tanchuma, explains it
Far be it from You to do a thing such as this, to put to death the righteous with the wicked so that the righteous should be like the wicked. Far be it from You! Will the Judge of the entire earth not perform justice?"
כהחָלִלָה לְּךָ מֵעֲשׂת | כַּדָּבָר הַזֶּה לְהָמִית צַדִּיק עִם רָשָׁע וְהָיָה כַצַּדִּיק כָּרָשָׁע חָלִלָה לָּךְ הֲשֹׁפֵט כָּל הָאָרֶץ לֹא יַעֲשֶׂה מִשְׁפָּט:
related post: http://kallahmagazine.blogspot.com/2015/10/dvar-yehudah-parsha-points-from-my.html