Charles Ellicott Commentary Deuteronomy 32:50-51

"and die in the mount whither thou goest up, and be gathered unto thy people, as Aaron thy brother died in mount Hor, and was gathered unto his people: because ye trespassed against me in the midst of the children of Israel at the waters of Meribah of Kadesh, in the wilderness of Zin; because ye sanctified me not in the midst of the children of Israel." — Deuteronomy 32:50-51 (ASV)

And die in the mount ... as Aaron your brother died in mount Hor ... because you trespassed against me. —It may be asked why Moses and Aaron should both have been made to ascend a mountain to die. I believe a clue to the reason may be found in the words and act which constituted their transgression. They were commanded to speak to the rock in Kadesh, and they struck it. The words which Moses used on that occasion were, Hear now, you rebels; must we fetch you water out of this cliff (Selagh)? The last words of the sentence are emphatic; and the rock is described as a cliff, not by the name given to the Rock in Horeb (Tzûr). The emphasis placed on these words has been much discussed by Jewish commentators, though it escapes English readers.

I suspect that the mistake Moses and Aaron made, in thinking it necessary to strike the cliff, also led them to think it necessary to ascend it, instead of gathering the congregation together beneath it, and speaking to it from below. This view harmonizes with the spiritual significance of the act. The smitten Rock in Horeb was Christ; the Cliff not to be smitten in Kadesh pointed also to Christ, ascended now, needing only the prayer of faith to call down all that He will give. And so Moses himself taught, in some of his latest words. It is not in heaven that you should say, Who shall go up for us to heaven, and bring it to us? ... But the word is near you, in your mouth.

The impatient words of Moses, after toiling up the cliff with his brother Aaron, had to be answered for by their ascending Mount Hor and Mount Nebo to die. Moses, as the more responsible of the two, had to ascend on each occasion, for his brother’s death and for his own. The remembrance of his brother’s death in the Lord may well have comforted Moses in the prospect of his own.