Charles Ellicott Commentary Exodus 32:21

"And Moses said unto Aaron, What did this people unto thee, that thou hast brought a great sin upon them?" — Exodus 32:21 (ASV)

Moses said to Aaron, What did this people unto thee? The second step was to inquire how the idolatry came about; and here Moses very reasonably addressed himself to Aaron. Aaron had been left in charge of the people (Exodus 24:14), to advise them, direct them, control them, if necessary.

How had he acquitted himself of this charge? He had allowed the people to commit a great sin. What excuse could he offer for his conduct? Had the people injured him in any way? The question is asked ironically.