2 of the priests placed fire not from God on the Altar. Because it was fire from man and not of God, the Lord devoured them with fire, just as the sacrifice on the altar had been devoured. The brothers of the 2 the Lord judged and punished were not allowed to mourn. How terrible! Yet, for them to mourn would have been a slight upon God. Just as the fire of man was a slight, feeling sorrow toward those previously judged is not our place. We are not the judge or the jury. We are the obedient servant. Get off your horse and obey.
God gives instructions to Moses for the role of the priests. They are to avoid strong drink, particularly in the temple. Why this is mentioned here I'm not sure, but it seems important. The priests are to discern the difference between holy and unholy and clean versus unclean. How can we discern as men what God deems as correct? Only through an understanding of the Word! This understanding is also important for the priests to fulfill their most important role, to each the children of Israel the Statutes that were given by Moses.
Aaron does not eat the Sin Offering but burns it and Moses, in his worry of mishandling the Word, harps to Aaron. Aaron speaks with knowledge Moses did not have. Aaron recognized his shortcomings on that day "such things have befallen me" - verse 19. He did not deem himself worthy to administer the offering correctly. In his way, Aaron moved the Sin Offering to a Burnt Offering. He recognized that God was almighty and still deserved the sacrifice, but He was in no place to complete the task. In his fealty as man he gave the power to God. This is the correct move we as men (and women) should all make. Give our trust to God, even as we fall short.