How did Moses sin?

2 Now there was no water for the congregation. And they assembled themselves together against Moses and against Aaron. 3 And the people quarreled with Moses and said, “Would that we had perished when our brothers perished before the Lord! 4 Why have you brought the assembly of the Lord into this wilderness, that we should die here, both we and our cattle? 5 And why have you made us come up out of Egypt to bring us to this evil place? It is no place for grain or figs or vines or pomegranates, and there is no water to drink.” 6 Then Moses and Aaron went from the presence of the assembly to the entrance of the tent of meeting and fell on their faces. And the glory of the Lord appeared to them, 7 and the Lord spoke to Moses, saying, 8 “Take the staff, and assemble the congregation, you and Aaron your brother, and tell the rock before their eyes to yield its water. So you shall bring water out of the rock for them and give drink to the congregation and their cattle.” 9 And Moses took the staff from before the Lord, as he commanded him.10 Then Moses and Aaron gathered the assembly together before the rock, and he said to them, “Hear now, you rebels: shall we bring water for you out of this rock?” 11 And Moses lifted up his hand and struck the rock with his staff twice, and water came out abundantly, and the congregation drank, and their livestock. 12 And the Lord said to Moses and Aaron, “Because you did not believe in me, to uphold me as holy in the eyes of the people of Israel, therefore you shall not bring this assembly into the land that I have given them.” 13 These are the waters of Meribah, where the people of Israel quarreled with the Lord, and through them he showed himself holy (Num 20:2-13).

Commentators puzzle over this passage. What exactly was the sin of Moses? The punishment seems to be disproportionate to the infraction.

i) Some commentators suggest it was sacrilegious to strike the rock inasmuch as the rock symbolized God. However, Moses was commanded to strike the rock in the parallel episode (Exod 17:5-6), so there's nothing inherently sacrilegious about that gesture.

ii) Moses is given specific instructions, which he disobeys. It's important for religious leaders to be just as accountable as the rank-and-file. Moses is not above the law. He must obey all God's commands, just like ordinary Israelites. 

iii) There's a sense in which many of the purity codes were artificial. Their significance is symbolic. It's wrong to disobey them, not because they are intrinsically obligatory, but because we have an intrinsic obligation to obey God. In addition, the symbolism was purposeful. So, by the same token, it's important for Moses to respect the emblematic connotations of his actions.

iv) By striking the rock without divine authorization, Moses was acting like a pagan sorcerer–as if he had the inherent power to extract water from rock. Using the rod as a magic wand. His action made himself appear to be the source of miraculous water, rather than God. That's probably the gravity of his offense. Had he been divinely commanded to strike the rock, the effect would be attributable to God, not Moses.