Charles Spurgeon Commentary Exodus 32:26-27

"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." — Exodus 32:26-27 (ASV)

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.

This is the man who pleaded for them on the top of the mount.

See how he acts in the sight of their sin; by divine authority, he strikes them right and left. Possibly, those who were killed were the men who refused to drink the water on which the powder had been sprinkled, or those who continued in rebellion against the Lord.