Why did Moses have the sons of Levi run about with swords slaying family and friends?

Today's OT Reading for CoE Common Worship was Exodus 32:15-34. Although to the modern ear it appears indiscriminate no doubt it wasn't. Please could someone explain why the Levites as the Priestly tribe were the only ones told to run about with swords slaughtering their brothers, friends and neighbours? Was this somehow an act of atonement through the shedding of blood? Also how does by so doing ordain them for the LORD's service? Best I can fathom is that, 1. That judgement had to be dispensed on the people for their stubborn idolatry. 1. That as the Priestly tribe the Levites having had the greater responsibility for leading the other tribes in righteousness, were required to dispense the judgement both as a punishment for having failed to lead the people, as well as priests conducting a redeeming blood sacrifice of atonement. However if that is the case then why would the LORD further punish the people at a later date? How are two punishments just?