While Moses is up on Mount Sinai for forty days and forty nights listening to God's demands for ostentatious furnishings, the people are getting restless. Where's God? We wanna worship him! He's not here. Gotta worship something!
Aaron spearheads the initiative to melt down the gold hoop earrings worn by mostly everybody ("Oh no, you did-n't) and fashion them into a golden calf. That makes sense. When Moses returns from the mountain and sees his people having a MASSIVE DANCE PARTY around this new idol, he is righteously pissed and smashes the tablets on which the Ten Commandments are written.
I'm sure his people get the point; they've been busted. But Moses is the kind of guy who likes to hammer home the point (just like God - no wonder they get along famously). So he burns the golden calf, grinds it into a fine powder, mixes it with water and makes everybody drink it! That is really mean. It sounds like something a disturbed mother with Munchausen syndrome by proxy would do.
Now here's the best part. Basically, Moses says something to the effect of, "If you're with us, you're against us". Some of his people (specifically, the sons of Levi) come over to his side and he tells them, "Put every man his sword on his side, and go to and fro from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbour" (Exo 32:27). Kill everybody! And they do! They kill three thousand people! God will neither stand for this business with the golden calf nor for this DANCING.
This is ridiculous. Moses spent all that time and energy hauling their asses from Egypt just to slaughter the majority of them.