Moses saw that the people were running wild and that Aaron had let them get out of control and so become a laughingstock to their enemies.
References for Exodus 32:25
So he stood at the entrance to the camp and said, “Whoever is for the LORD, come to me.” And all the Levites rallied to him.
Then he said to them, “This is what the LORD, the God of Israel, says: ‘Each man strap a sword to his side. Go back and forth through the camp from one end to the other, each killing his brother and friend and neighbor.’ ”
References for Exodus 32:27
The Levites did as Moses commanded, and that day about three thousand of the people died.
Then Moses said, “You have been set apart to the LORD today, for you were against your own sons and brothers, and he has blessed you this day.”
The next day Moses said to the people, “You have committed a great sin. But now I will go up to the LORD; perhaps I can make atonement for your sin.”
References for Exodus 32:30
So Moses went back to the LORD and said, “Oh, what a great sin these people have committed! They have made themselves gods of gold.
References for Exodus 32:31
But now, please forgive their sin—but if not, then blot me out of the book you have written.”
References for Exodus 32:32
The LORD replied to Moses, “Whoever has sinned against me I will blot out of my book.
References for Exodus 32:33
Now go, lead the people to the place I spoke of, and my angel will go before you. However, when the time comes for me to punish, I will punish them for their sin.”
References for Exodus 32:34
And the LORD struck the people with a plague because of what they did with the calf Aaron had made.
References for Exodus 32:35