What did this people unto thee? Moses does not suppose that the people had really done anything to Aaron. He asks the question as a reproach—they had done nothing to thee—had in no way injured thee—and yet thou broughtest this evil upon them.
So great a sin. Literally, "a great sin"—the sin of idolatry. If Aaron had offered a strenuous opposition from the first, the idolatry might not have taken place—the people might have been brought to a better mind.