When Moses reached the bottom of the mountain, he heard singing and saw revelry around the calf, and he dropped the two tablets and they shattered. He ground up the calf and made 'bitter waters' out of it, which he forced the people to drink, and then he turned on Aaron. And God, we are told, was so angry that he could have just about killed Aaron.
"Who is on God's side?" (Exodus Chapter 32 verse 26)
Moses asked, and the Levites stood beside him. He then ordered them to
"kill your brother, friend, and neighbor." (Exodus Chapter 32 verse 27)
He then announced that this was the day of the institution of the Levitical priesthood because by killing their brothers, friends, and neighbors,
"you have brought a blessing this day upon yourselves." (Exodus Chapter 32 verse 29)