Deu 9:17-21 So I took hold of the two tablets and threw them out of my two hands and broke them before your eyes. Then I lay prostrate before the LORD as before, forty days and forty nights. I neither ate bread nor drank water, because of all the sin that you had committed, in doing what was evil in the sight of the LORD to provoke him to anger. For I was afraid of the anger and hot displeasure that the LORD bore against you, so that he was ready to destroy you. But the LORD listened to me that time also. And the LORD was so angry with Aaron that he was ready to destroy him. And I prayed for Aaron also at the same time. Then I took the sinful thing, the calf that you had made, and burned it with fire and crushed it, grinding it very small, until it was as fine as dust. And I threw the dust of it into the brook that ran down from the mountain.
Moses broke the tablets, “Not by an unbridled passion, but in zeal for God’s honour, and by direction of God’s Spirit, to signify to the people, that the covenant between God and them contained in those tables was broken and made void, and they were now quite cast out of God’s favor, and could expect nothing from him but fiery indignation and severe justice.” (Poole)
When he saw the sin of Israel and knew the holiness of God, Moses was very afraid for the sake of the people of Israel. Aaron’s sin was so bad, that he surely would have been destroyed by the LORD – except Moses prayed for him. This shows both the prevailing power of Moses’ prayer and the great love in the heart of Moses. Moses burnt the idol, ground it up, and sprinkled it in the people’s drinking water for three reasons.To show this god was nothing and could be destroyed easily. To completely obliterate this idol.To make the people pay an immediate consequence of their sin. (Guzik)
And I took the two tables, and cast them out of my two hands,…. In wrath and indignation at the sin they were guilty of: and brake them before your eyes; as an emblem of their breach of them by transgressing them. Because of all your sins which ye sinned, in doing wickedly in the sight of the Lord, to provoke him to anger; for they were guilty of more sins than one; besides idolatry, they were guilty of unbelief, ingratitude, &c. which were notorious and flagrant, were done openly and publicly, in sight of his glory and majesty on the mount; all which must be very provoking to him, and on account of these Moses prayed and fasted. (Gill)
A proper view of the sinfulness of our sin(s) should humbly drive us to our knees.