Aaron, when asked by Moses why he has done this thing, tells a lie:

And Moses said unto Aaron, What did this people unto thee, that
thou hast brought so great a sin upon them?

And Aaron said, Let not the anger of my lord wax hot; thou
knowest the people, that they are set on mischief.

For they said unto me, Make us gods, which shall go before us:
for as for this Moses, the man that brought us up out of the
land of Egypt, we wot not what is become of him.

And I said unto them, Whosoever hath any gold, let them break
it off. So they gave it to me: then I cast it into the fire,
and there came out this calf.

And when Moses saw that the people were naked; (for Aaron had
made them naked unto their shame among their enemies:)

Then Moses stood in the gate of the camp, and said, Who is on
the Lord's side? let him come unto me. And all the sons of
Levi gathered themselves together unto him.

And he said unto them, Thus saith the Lord God of Israel, Put
every man his sword by his side, and go in and out from gate
to gate throughout the camp, and slay every man his brother,
and every man his companion, and every man his neighbour.

And the children of Levi did according to the word of Moses;