Or HaChaim on Numbers 20:12/אור החיים על במדבר כ׳:י״ב

1 ויאמר ה׳ אל משה, G'd said to Moses, etc. Why does the Torah switch from using the relatively harsh וידבר in verse 7 where it did not appear called for, and now portrays G'd as speaking softly, at the very moment when Moses had become guilty of a transgression? It seems ludicrous that at the moment when G'd decrees a harsh penalty on both Moses and Aaron the language this is couched in is most friendly! We may understand this in light of what Bamidbar Rabbah 19,12 writes that Moses said to G'd that seeing He had decreed that he would die in the desert together with all the people who had believed the report of the ten spies, subsequent generations would conclude that he too was no better than they. He therefore pleaded with G'd to record his sin in the Torah to make certain no one would think Moses was guilty at the time the spies returned. This is the reason the Torah wrote the words יען לא האמנתם בי. Thus far the Midrash. The reason that G'd introduced the paragraph dealing with Moses' punishment with the soft ויאמר was to draw our attention to the difference between what had caused the death of the generation of the spies and that of Moses and Aaron respectively. On no account were these leaders to be compared to their flock although they shared the fate of dying in the desert.