Charles Spurgeon Commentary Exodus 32:25-28

"And when Moses saw that the people were broken loose, (for Aaron had let them loose for a derision among their enemies,) then Moses stood in the gate of the camp, and said, Whoso is on Jehovah`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 Jehovah, the God of Israel, Put ye every man his sword upon his thigh, 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 neighbor. And the sons of Levi did according to the word of Moses: and there fell of the people that day about three thousand men." — Exodus 32:25-28 (ASV)

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:

The rebellious, the idolatrous, the men who had defied the authority of God, were to be summarily executed on the spot.