So Moses stood at the entrance to the camp and said, “Whoever is for the LORD, come to me.” And all the Levites gathered around him.
Moses begins with Aaron, not ordering his death like the ringleaders, because there is a world of difference between those who rush headlong into sin and those surprised by infirmity into it. Aaron deserved to be cut off, and would have been, except Moses interceded for him; now he calls him to repentance, showing him he hardened their hearts and strengthened their hands in idolatry.
AI summary
Commenting on Exodus 32:21-29
And he said unto them, thus saith the Lord God of Israel,.... The following orders are given by Moses, not of himself the chief magistrate, and as the effect of heat and passion, but there were from the Lord, who was Israel's God and King; he had them expressly from him, or by an impulse on his spirit, or in such a way and manner...
Moses stood in the gate of the camp, and said--The camp is supposed to have been protected by a rampart after the attack of the Amalekites. Who is on the Lord's side? let him come unto me--The zeal and courage of Moses was astonishing, considering he opposed an intoxicated mob.