From the book of Exodus
Now when the people saw that Moses delayed to come down from the mountain, the people assembled about Aaron and said to him, "Come, make us a god who will go before us; as for this Moses, the man who brought us up from the land of Egypt, we do not know what has become of him."
Aaron said to them, "Tear off the gold rings which are in the ears of your wives, your sons, and your daughters, and bring them to me."
Then all the people tore off the gold rings which were in their ears and brought them to Aaron.
He took this from their hand, and fashioned it with a graving tool and made it into a molten calf; and they said, "This is your god, O Israel, who brought you up from the land of Egypt."
Now when Aaron saw this, he built an altar before it; and Aaron made a proclamation and said, "Tomorrow shall be a feast to the LORD."
So the next day they rose early and offered burnt offerings , and brought peace offerings; and the people sat down to eat and to drink, and rose up to play.
Looks like everybody thought that Moses was dead or lost, so the people decided to take destiny into their own hands. And it appears that even aaron had the same opinion. Though the idea wasnt originally his, he was very much into it. It even appears that the idea of what god to make was his. It appears he intended to take the place of Moses.
And we all know that Moses massacred many people who partook in this episode. The question is, why didnt he hang the ringleader who happens to be his brother?